I think FPGAs are the only way;
You're going through the 5 stages of "no CPUs with external memory busses grief" that I've gone through.
Based on my research the only vaguely viable options of parts for which a lot of NOS exists is
* I960
* 68k (with 24 bit memory bus)
Based on my research (and I really did spend months looking at options) those are the only available chips with external memory busses and capable of supporting an MMU.
And they'd still need an FPGA as chipset
