Andrew,
Is the use of the 8255 your own design or did you lift this from an MSX2 implementation? All the MSX2 implementation schematics I can find use an 'MSX2 Controller' IC which I assume is a custom chip.
I had a pretty bad failure on my baseline development system which has set me back a few days. It looks like I fried my floppy controller (specifically the FD1771 chip). Last week I had a power supply failure which lead to me finally replacing the old transformers/Caps with switching supplies.
-Neil