Just to follow up on this, I did find a Centris 610 and it even came stock with a ROM SIMM slot complete with ROM SIMM. As a bonus, it works with dougg3's SIMMs. FWIW, the Centris/Quadra 610, 650, and 800 all share the djMEMC memory controller, so this should be applicable to all of the above, so in theory one could populate the ROM SIMM solder pads on those machines and have this work.
With a fairly minor tweak to the ROM (plus disabling the ROM checksum test, and RAM test), I've got the Centris 610 seeing 128MB SIMMs (the 610 only has 2 RAM slots)
