It's a neat idea, and a proven one, it was done in at least a couple case in the past. There is a book on Underhammers that was written back I beleive in the 60's and it was and probably still is the definitive work on the subject, there are some good pictures and a few line drawings of such a lock. It was actually recreated in the 1960's by a pretty well known target shooter using a coil spring setup.
If you did it make sure it has a long enough tang that could be screwed to the breech tang, this would secure it at that end, the forward portion would actually be further secured by a special nipple arrangement that locked it to the barrel, much like a washer around a bolt. This setup would also contain a large portion of the flash from the percussion cap. I think you would be well served to experiement with this system, you can easily create a set trigger arrangement with it as well. I don't think if done properly that it would weaken the stock, just have to remember to inlet it carefully and take no more wood away than is necessary and it should work fine.