Figured out my problem: The AboveBoard ROM by default was calculating
the wrong address for the start of its memory (it was top of planar
memory + 128k, creating a hole which obviously would cause problems).
The AboveBoard option diskette has a DOS utility (softset.exe) that lets
you adjust this. I've done so and now everything seems to work...
Yay!
Thanks for all the helpful suggestions!
Josh