Ideas

[BEETLE]

My arms are itchy now after looking at those pics

[ecduzitgood]

Cut access holes to install the bolts through the side (like in the photo you are pointing at), then just fiberglass the access hole after installing the railing. You can keep all the work above the rub rail which is area you will be painting anyway, right? (so make the holes big enough to make it easier to install the nuts). Just pre paint where the mounts sit before install the railing, once you insure you get all the nuts on, finish the glass work tape the mounts to protect from paint scuff and apply the final coat, don't forget to tape the rub rail and scuff the whole area where the paint will go to make sure you get good adhesion. I would also look into the proper cleaner to prep the area before painting to eliminate flaws with the paint like fish eye's, once cleaned/prepped do not touch where the paint will be going and if you have, wash that area again. It's been a while but when I used to prep for paint I used a fast evaporating reducer (3812 at the time) and wiped with one hand to put it on and had a dry rag in the other to wipe it off (Caution watch out for solvent soaked rags they can spontaneously combust if left in a pile, put them in an open metal container and place it where it won't cause damage if it does catch on fire).

[Mr. Sandman]

Good JOB! I think it looks very solid. That should out last the life of the boat. Your bow rail is probably stronger than your cleat now!
Just take care when you drill and tap so you get a straight hole. You can work on my boat anytime. I have several projects in mind that require that kind of destruction/rip out. I want to put in a second door in the pilot house and put in a transom door. Both will require substantial beefing up to maintain integrity.