So many things have been "tossed around" in this thread that Mike's actual system requirements, i.e. the characteristics without which his system won't meet his needs, are unclear.

I haven't seen any data regarding (1) what kind of program store (part number, speed grade, etc.) he's using, (2) what kind of data memory he's using, and so on. It's possible he could use memories fast enough to allow stealing a clock tick or two for off-board access.

If I were building this thing, I'd use a cheap CPLD to provide the logic, such as an ALTERA EPM7128S. These are available through distribution for about $20, or, frequently, on eBay for about $3. If one doesn't like the ALTERA part, XILINX makes equally capable (XC9500-series) CPLD's in PLCC-84 packaging, which package is very "prototypable." The key is that this CPLD type is JTAG programmable in-situ. I'd do this because I'm pretty confident that I can "bring it off." Since most of the hardware is data/address path steering and multiplexing, there's no reason it shouldn't be done in discrete MSI/SSI (74xxx-series) logic. The programmable logic simply allows one to "rewire" or otherwise modify the circuit without using wire or soldering iron.

RE