Yes, the CPU is soldered on on all Tanzania clones. However, there are G3 upgrades available that plug into the L2 cache slot, that are designed just for the Tanzania based Macs.
I have a Clone, a PowerComputing PowerTower 200e, but mine's a Catalyst with a CPU slot (thank the lord ) and 320MB RAM, oh and a G3/350. Not tried XPostFacto on it yet, but seeing as it works ok on the 8550 then it ought to work on the PowerTower too.