Possible options:
1. There is an exchangable CPU card with various type of CPU is VSPA from SiliconBandwidth / Panda / Rock City for Pentium II / 400 MHz in 1998.
"The Panda Project, Inc. Monday introduced the Rock City ST-400, the latest addition to its award-winning line of stylish Rock City personal computers. The Rock City ST-400 features an Intel 400MHz Pentium II Processor with MMX Technology, AGP support and 100MHz bus capability, at a base price of $2,595. Panda expects to introduce advanced Alpha-based models and upgradeable models of Rock City that incorporate VSPA and Compass technologies to extract additional performance at a very low price point in a small form factor."
2. Multicomputer standard from Ziatech / PICMG standard, but Ziatech was bought by Intel.
3. Perhaps you can use HTX bus/connector, because both AMD Opteron and PowerPC G5 support Hypertransport.
http://www.hypertransport.org/tech/tech_specs_conn.cfm4. SlotServer technology.
http://www.south14.com> No porting
Do you use PowerPC CPU card with the buggy MAI Logic chipset?