According to an interview with one of the upgrade company CEO's, the cache slot on the 7200 "doesn't have enough wires." In the Developer's Notes on the 7200, it refers to the cache slot as a SIMM slot, not a DIMM. I know that the same cache chips work in the 7200 as the x500 machines. I haven't been able to reconcile those facts.
Ultimately, we 7200 owners have to accept that our machines will never have a G3 card manufactured for them. :-(