2)Kai's address decoding scheme works but it also means that I would have to sacrifice the entire 256 byte page for every 8-bit I/O port added.
that is why MMIO is generally done by decoding A0-A7 ANDed with A8-15 making ff00-ffff I/O space.

3)I have an old device that multiplexes P1 for connecting to one 74HC244 and one 74HC573, now I am moving this multiplexed port to P2 and experimenting the new scheme.
See above, you MUST keep in mind tht using the 'memory ports' 0 and 2 require special logic.

4)One other thing that I am afraid of is that this type of unconvensional scheme may upset any emulator. I am using Signum System's ICE and have already gotten enough headaches.
if your emulator can operate with RAM external to the emulator (most can) it will be no problem.

Erik