With the memory map of your SBC being as simple as it is, it may be beneficial to hack together a quick and dirty PC emulator for it, and then trace the basic assembly code as it starts up. That may help you narrow the problem down quite a bit.
I am more interested in the SBC, but I would buy the full AltairPC if the price was right. (would most likely just put a backplane of some sort in it and connect to the SBC) Would it be possible to include a DEN line one the expansion port (like in the micro-kim) so that one could fully decode the I...
Your original idea sounds good to me. Keep it simple, and allow for an expansion connector. That way, if the end user wants S-100 compatability (or another bus) in the future an interface card could be designed.