Sounds like a great idea to me. Answer to drainage is, depends on what mix your using and what your planning to grow. I would leave the drain about half open (screened) and put a layer of something that won't rot on the bottom, I use packing peanuts. Maybe you could figure out a way to plug the drain from the outside and adjust the drainage if there turns out to be too much drainage, kind of a plan B. I would also mound my mix at least 3 or 4 inches above the rim of the tub. This is assuming it's a "true" bog for CPs/orchids. If it's for marginals just leave the drain plugged and fill it w/ muck.

Have fun!

Thanks KWoods. I plan to put aquarium rock on the bottom since I can get all I want for free. I was just afraid of overflow from hard rains and thought the drain would be better open. I guess I could always plug it up from the outside if it drains too freely. I'll be putting marginals in it...the overflow of my other bog.