Sure, why not? You're basically making a pickguard of wood built right into the body so it should all go together fine, pretty much as it was designed in the first place. I wouldn't route the whole back out though, just what's needed to get the pickup in there and a single rectangle route just deep enough for a cover plate. I'd start with about .2" face thickness and see if the pickup seems to mount ok or not. I'd try to stay at least .125" final thickness depending on the wood you're using. You could also see if the pickup will mount in a pair of pickguards stacked up and measure that for reference... Better yet, do a mock-up with some scrap wood and you'll know exactly what to expect when you get ready to cut the actual body.
Good suggestion on the routing. I can definately do that, I'll just make three templates. One with three openings for the pickups, triangle shaped, to go from the back, one rectangle to route out for the cover plate and one with the openings from the front. .2" sounds easy, .125" will be delicate but I can do it. I should probably route the pick up openings in the front first, then the pickup openings from the back and then the rectangle for the cover.... seems like the easiest way to see the face thickness I have left.
The wood is ash, and I will definately try all this on scrap first. I am also making two of everything as I go so first I have been testing a setup on my 'practice' neck or body, making adjustments and then doing the operation on the 'good' version. I figure with this stage, and with fitting the neck, I'll do a test run on scrap, then on the practice and then on the good one. Can't hurt.
I am using a hard tail bridge. It might be possible with a tremello, I think it would depend on how thick the body is....
Thanks for the help!