G'nite. [
] I see it hasn't been mentioned yet so I'll chime in even though you're now on the better trail for your hunt. The card o posted is for the040 PDS slot, not NuBus which is too slow for internal memory I/O outside of RAM Disk operations.
You've inadvertently (or through a terminology mixup?) posed and interesting question though. I wonder if putting Cache on the LC PDS might be worthwhile? The LCIII extension to that slot has a CPUCLK input so it should at least theoretically support Cache operations? If you can't find a Cache Card for that slot in the historical record it shouldn't be hard to create one. You've made my MIA Quadra 630 board's PDS quite hungry. It would be fun for the CC crowd as well, especially the Mystic modders?
CPU socket's not buffered like the PDS, so development would be a bit more difficult, but should be doable as well in this day and age. It wouldn't have made much sense back in the day to hop up lowend machines in this manner from a business perspective.