John I know how well you plan projects and it already sounds like you have addressed many of the issues. Some other items:
1. Acoustical insulation in the ceiling. Its amazing how those long wavelengths make bass as loud upstairs as in the HT. I've gone through all sorts of Safe n' sound Roxul insultation from Home Depot.
2. Wiring. I'd run a 20 amp line into the room. Over time power requirements seem to get larger and amps bigger so its an easy one to deal with now. Location of components and speakers needs to be thought through if you are going to hide much of the cabling in the wall. Use 12awg or higher for all run lengths to cover future contingencies.
2. Construction Dust.
Bad for all electronics not just projectors. I would definitely keep all equipment covered when not in use. Better still don't leave electronics in the room until you are finished the project.
3. Ventilation ducts. Is amazing how much resonance there is in those things. In order to minimize it I would wrap them in insulation as well before enclosing. Also try to put the vent openings away from your mains or subs.
4. Seats. How big is the screen. At 1.5X the diagonal is your room big enough or do you want a smaller screen.
5. Risers. These things can really cause boomy bass especially for the seats on the riser. They actually act like large Helmholtz resonators. My solution was to stuff them with...yup...more Safe n' sound insulation to minimize their impact. By doing so they start to act like a bass trap lying on the concrete floor. Some guys I know actually drill 3" wide "ports" in the back of them for more sound but I would not recommend it.
Good luck with the project. If you buy the right kind of beer I'd be glad to come over when you need a hand.
