I wouldn't say there is necessarily anything wrong with using modern materials such as Accraglass, but the question is why? If you are worried about protecting the stock, then why not just seal it with a sealer like Permalyn? I do use Permalyn to seal my stocks, and get it everywhere I can... Under inlays, barrel, lock and butt plate... Everywhere. And Permalyn is a modern material. I think this is especially important under inlays, since all are fitted tight enough to hold water with capillary force. It makes me wonder if some of the ancient makers made round-bottomed barrel channels, just so the water would drain, and not be held there except at the corners of the barrel. I am pretty certain though, that after a rainy hunt, the 18th century man didn't come home or to camp and remove his lock, barrel and inlays just to dry them.
I think it is really more of your personal preference. I use Permalyn sealer because it makes me feel like my stocks are better protected, but I have absolutely no empirical evidence of that. I also use a bandsaw to profile my stocks. I make use of lots of electric lighting when making a gun, and I even listen to the radio when doing it. I even go so far as to use chisels and files made of modern steels, and drills that are have little in common with those they had in the 18th century.
So where do we set the limits? Where do historical correctness and modern methods intersect? The plank of wood you are about to turn into a gunstock, I can almost guarantee, was not cut in a pit saw, but in a modern mill. Are you going to go through the pains of a hand-rubbed BLO finish, or are you going to settle with something like Danish Oil? I used BLO on my first few guns, and frankly, I find you can get modern finishes that look almost exactly the same, but are much more durable, so I choose to use those. And after that, sacrilege of sacrileges, I hit the whole gun with a coat of SC Johnson paste wax after that. Of course, they had waxes back in the day, but not petroleum-suspended SC Johnson.
IMHO, you should do what satisfies you if it is a gun for your personal use. If it is a gun for sale, represent it accurately and honestly. Accraglass is great, and has it's uses. Smart dog mentioned he finds it useful in his work, and his is excellent. Mike Brooks, on the other hand, who also does excellent work, has his very valid opinion too. I guess it is like when you go to the grocery store, and the bag-boy asks you, "Paper or plastic?".
Matt