After much procrastinating due to worrying about doing it wrong, on Sunday I cut the wiring harness and replaced the right front with the Porsche ABS wiring harness repair kit. I was first able to prove the problem was in the RF harness by letting the car idle and then wiggling the loose harness in the wheel well. ABS light went immediately on when I did that, with the expected ground/short error code.
First I unwrapped the cloth wrap from the bundle that runs along the cowling (from the battery area / wiper assembly area, to the hole in the top of the fender). The bundle also has a couple of EVAP canister related wires that are no longer connected (race car), so I moved them out of the way and they'll now sit idle inside rather than outside the fender wall. Then I carefully cut & peeled away the thick heat shrink, to reveal the individual wires and the factory solder points. They're all sealed with some kind of clear epoxy to make them waterproof. I was worried that in manipulating the heat shrink off, I had nicked a couple of the wires but it end up to be OK (and I'll tape over the nicks in the insulation). I had to decide whether to cut before or after the factory splice, and opted to cut a few inches after, in order to preserve more factory wiring in case of a need to redo my work. It's also easier to work near the edge of the car than the center.
I did my best, which is not great, to cleanly strip, crimp, and solder together the new harness wires on to the old ends. And again, in case it didn't work, I left extra slack. I just did the common ground, and the +/- ABS leads, leaving the brake pad wear sensor +/- leads disconnected since I don't (can't) use wear sensors on the race car. Wiggled the (new) harness, no ABS light. Test drive, no ABS light. And then on Monday took the car to Laguna Seca to test & race, and the ABS worked correctly and the warning light never lit up. Finally, I think my next real race weekend I can brake with confidence.
Now I'll go back in there and clean up my work! (Cut the new harness leads back to a better length, and do a better job on the soldering, and then apply the new heat shrink.)