Aha. Since LDW/LDB/STW/STB were changed to shunt around memory buffering for testing purposes, they weren't setting cpustate->ramaddr. SBK relies on cpustate->ramaddr. Now that they set ramaddr properly, there's a slightly larger amount of happiness:

The firework particles at the beginning of Vortex now show up properly:

Also, Stunt Race FX is considerably improved, including the tires now rotating the right direction on the car-select screen, and actually being able to get in-game and play. There are still some issues, though: The cars keep "popping" upward in the car-select screen, and bits of the scene geometry keep dropping out.