Success! After installing DIOS MIOS Lite 2.4 (NAND) and updating to USBLoaderGX 3.0 r1203, SD-ripped GC games boot and play just fine now, as it did before.
I'll try Devolution and DIOS MIOS out later just to compare load times and stability.
Thank you very much, Fishaman P, The Real Jdbye...