Summary: It kinda worked. I get garbled video with the camera and on bootup after the Samsung Galaxy animation.
I started on a semi-recent CM7.1 Nightly (90-ish).
I flashed to stock using AIO Captivate Toolkit. That got me to JF6. Note all download modes are initiated using USB jig.
As soon as I verified a good boot into stock, I fired up Heimdall One-Click, put the phone into Download mode and clicked the magic button...
It flashes, reboots, but then gets stuck at the white AT&T boot screen. No progress for many, many minutes. I get desperate and tried to shut it off. In the process, the phone reboots and begins what appears to be a normal boot process. This boot completes, but I'm left with the aforementioned artifacts - the camera doesn't work and I get garbled graphics on boot. Also, the phone feels laggy as heck.
I'm guessing I may have prematurely interrupted the initial boot process. To be honest, I wasn't tracking time, so I don't know if it was only 5 minutes or 15. I just know it didn't seem right and much, much slower than flashing to stock JF6 was.
I'm going to try a more conventional Odin flash to see if I get better results.