I have a large piece of cherry driftwood that just won't sink. It's been soaking for over 2 months now. It's very heavy, and required a piece of 12"x12"x1" tile slate to keep it submerged. I can't sink the screw, it cracks the slate in half. I can't put the slate w/a large screw on the bottom of the tank, I'm afraid the weight/pressure would crack the bottom of the tank. . Thinking eggcrate would cushion it enough to prevent that. I have a 75 gallon shell dweller/tropical set up with over 200 shells in the bottom of it. I'd like to tie some java fern to the shells to add some color to the tank.
I really don't think that the screw would pose a threat to the glass on the bottom of the tank. If you have a hole going through the slate you could use a vinyl screw instead of the metal one...they come in every size. I had a piece of driftwood also that just wouldn't sink and it took about three-months so eventually it'll get there. But don't worry about the bottom glass. Especially if you have substrate.
Yes...if you can get a high speed bit through the slate without breaking it you could do that. Otherwise, you could temporarily use fishing line to tie the driftwood onto the slate or some other suitable rock to keep it wieghted down until it no longer floats.
I don't see a problem with the eggcrate. I got into the habit of using it when I had a tank full of African cichlids, because they'd dig all the way to the bottom. I have it under the substrate in my planted tank where I have rocks, because my Bolivian ram digs around them, and I don't want the rocks to suddenly one day crash onto the glass because he dug all the sand out from under them. I've had it in there for years with no issue that I have noticed.