Circling back, I tested out the output from my cab to the OSSC, and my particular setup doesn't play nice with it. I tested my standard setup, and I also tested bypassing the JROK sync cleaner, because the OSSC is known to not like/want a sync cleaner in the chain. In both cases, it did not see the signal.
Not to say that there might be some tweaking to be done on the OSSC to get it to work, but that's about as far as I care to take it.
It is also worth noting that with the sync cleaner removed from the chain, the DVDO VP-30 also did not like the signal. Its LED flashed like it was trying to do something with the signal, but apparently could not sync with it. I don't think I ever tested that when I was initially putting my setup together, just put the sync cleaner in the chain on JROK's recommendation, and got lucky that various device up to and including my current and final setup with the VP30 worked.
Wes - I saw your post on the Video Game Perfection forums. Sorry I can't be much help above and beyond testing my setup, which is fundamentally different from yours. I may pull the board out of the cab eventually and test with the Mike's inverter board->supergun->SCART output to OSSC, but don't have any immediate plans to mess around with out. Here's to hoping you find a quick solution to your problem.