Okay, I finally got my first test setup for this topic together (Nah, I was originally 'trying to pick the best Am5x86s but whatever).
So (post will be hopefully extended in the future):
Shuttle HOT-433 (ISA/PCI, UMC888x chipset): CPUCHK shows that the cache runs in WB mode.
Gigabyte GA-5486AL (ISA/PCI, ALI 1487/89 chipset): The same result, it's in WB mode too.
QDI V4P895 (ISA/VLB, OPTi 895 chipset): Works fine too.
MS-4138 (ISA/VLB, SiS 471 chipset): I couldn't test it. It didn't boot with the POD, not even at 63MHz, even though it should support it.
After 3/3 (working) boards reported WB mode I grew suspicious and looked made Speedsys graphs on them. All had breakdowns in the write/move speed after the L1 cache so I guess Chkcpu was right.
[Unfortunately none of my 3 PODs can stably run at 100MHz. Quake kills them all, it's a nice indicator.]
Shame on us, doomed from the start
May God have mercy on our dirty little hearts