Followup:
cpu0: 700.00 MHz, disabled 256KB L2 cache no parity parity enabled
Apparently it was this. With these options added to a custom kernel, it boots fine. I erroneously thought that a GENERIC kernel would simply leave the caches off if it didn't know better.
options L2CR_CONFIG="(L2CR_L2E)" options L3CR_CONFIG="(L3CLK_35|L3CKSP_4|L3PSP_0|L3RT_PB2_SRAM|L3SIZ_1M)" Thanks, John