Almost-A-SCAMP Progress Report #2-1 . . . .

[Charles Brennan]

Doug, Nothing like doing something, then describing it on a forum, only to have someone make a really good suggestion or comment that TOTALLY INVALIDATED EVERYTHING!!  That you had just painstakingly accomplished. 
Ah, well.

Thinking about Kruse'n's idea, I was reminded of the "Mastering Epoxy with Russell Brown, Part 1 - Introduction to Filleting" video:
https://www.youtube.com/watch?v=yDoauHaOfBQ
In it, at around the 2 minute mark, he demonstrates how an epoxy joint is stronger than the surrounding wood, by breaking a filleted joint.
Several other boat building forums claim the same thing about epoxy/wood being stronger than just wood, alone.

It occurred to me, that if one over-drilled a handrail hole to say 5/16" or 3/8", then filled the hole with thickened epoxy (I hasten to add: something structural, like 403 glue and NOT non-structural, like 410 micro-balloon fillers) then after it was cured, you could drill and tap as per Norm's description; you'd  have a much tougher and stronger pull-out force than just wood, alone.
I can't find anything on the web to calculate, or get a hard number of what that pull-out force might be, but I have to believe it would have to be somewhere between the 400 psi of Teak, versus the 600 psi of Stainless Steel ¼-20 nuts.

I'm already committed to my method, but you never know what I might scrounge up at a marine flea market, somewhere! 
If I trip over some fantastic deal somewhere on teak hand rails, prior to getting the cabin roof done, I might re-think the idea.

While I'm not hard-core enough to eschew ALL Nautical Proud-work for say, Starboard, or Stainless Steel hand rails, anything to help minimize on-going maintenance would be desirable.
After all, I MUCH prefer SAILING    on boats, to WORKING  on boats!   

Just an idea to consider,
Charles Brennan

[Krusen]

  Never one to dispute good research or different opinion, Charles has a winner.

On the other hand, I had some fine quality oak, and the tools to do it my way, and the function and appearance was good enough to fool some of my sailing friends and all of my neighbors.  From a short distance, it is the color of teak.  Strength wise, it survived my worst attacks going forward while heeled.  Not scientific, but practical.

I am surprised that teak pull out numbers are as high as they are.

There was no above deck unpainted wood on my Mac, the tiller pilot anchor reinforcement white to match the seat, flag base, oak with bronze paint.  Below deck, all varnished.

I expected DougSC to be more interested than Charles, but posted outside the Scamp thread, would easily fail to be noticed. Thanks for being friendly to the thread drift, Charles.

KRUSEN

[Noemi - Ensenada 20]

What are those bolts called, that have wood threads on one end, and nut threads on the other?  I had considered embedding those in the grab rails, pushing the nut-thread end into holes in the deck, then bolting them through a matching set of grab rails on the (I know it's not a ceiling, but that) inside.  Secured with a nut, or better yet acorn nut, the setup would provide grabbers inside and out.

If I ever get my boat out again.....

[Spot]

What are those bolts called, that have wood threads on one end, and nut threads on the other?  I had considered embedding those in the grab rails, pushing the nut-thread end into holes in the deck, then bolting them through a matching set of grab rails on the (I know it's not a ceiling, but that) inside.  Secured with a nut, or better yet acorn nut, the setup would provide grabbers inside and out.

If I ever get my boat out again.....

Hanger bolts
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[Charles Brennan]

Noemi,  Hanger bolts are not nearly good enough for a suspenders-and-belt kind of guy. (Who likes to Over-Engineer stuff!) 
A ¼-20 hanger bolt typically has a pull-out strength in Teak, of around 270 pounds when using a Grade 5 bolt.

That's less than half of the 600 pounds I expect from my ¼-20 through-bolting of my Teak handrails.
What helps make my approach (mostly!) bullet-proof, is that I used a Forstner bit to achieve a flat surface bottom for the round head bolt to sit on.  This means that the Janka Hardness rating of 1155 pounds of force for the Teak wood compressive forces surrounding the bolt, is also helping to counter the bolt's tensile forces. The Teak wood is 1155 pounds and the tensile strength of a ¼-20 grade 8, 316 Stainless Steel bolt is over 4000 pounds.  In my case, the weakest link in my chain of materials is the ¼-20 Nylock nuts of 600 pounds each, times 4 bolts, for a combined load of 2400 pounds.   
Had I used a counter-sink at the drilled hole at top of the hand rail and then used a flat head bolt as a fastener, (Look, Ma! No Teak plugs required!)    it would have split the hand rail, nearly as soon as I torqued down the nuts.

Kruse'n's interesting solution uses ¼-20 tapped threads directly into Oak, for a pull-out strength of 361 pounds. (Since Oak is not as hard as Teak.)
Had he not used coarse threads, and had (for example) used finer ¼-28 threads, the pull-out strength would have been reduced slightly, to 311 pounds. 

Due to the coarseness of the lag-side threads of your hanger bolts, the wood can only support a pull-out strength of 270 pounds.  Times 4 holes gives us slightly over 1000 pounds (1080 lb) for total loading.
The reason for this (after all, ¼-inch is a ¼-inch, no?) is that the hanger bolt's lag screw is partially fracturing and crushing the wood fibers, as it cuts into them, weakening the surrounding wood.  Kruse'n's tapping method cuts grooves into the side walls of the threaded area and does not weaken the wood, through crushing.

For your postulated application of back-to-back fastened grab rails on deck and over-head (ceiling) I would have recommended Forstner drilled holes in both sets of grab rails and a ¼-20 bolt and ¼-20 nut attaching them together, followed by teak plugs.

Stuff I do to my boats, is usually performed with the following question in mind:
"Would I bet my Life, on a given item: n holding up in extremis?"
Seen a lot of "in extremis" in my life, Chica.
Have had enough stuff fail on me, that should NOT have failed,    that I now beef up everything I touch, just because.
It's also why I never give a thought to the hand rails I installed on Urchin 45 years ago.
I've hung outrageous things on them, used them for tying off to stuff I had no business tying off to and yet, here they still are; because they started OUT being over-built.

I am shooting for this same peace of mind, on everything I am building onto the Irish Pennant.

Hope this clarifies,
Charles Brennan