Thought I should conclude with what I discovered.

First, I found a flight that had BusinessFirst availability at the time of booking, so that helped a great deal. I ended up booking the flight with the CO flight number [more on that below]. I was able to use my UA confirmation number to pull up the flight on Continental.com and apply my long-idle OnePass miles to my reservation and pay the co-pay for the upgrade. And then I could select my seat, so that was all good.

In my discussions with UA staff (who put me on hold for a nice long ten minute chat with their CO counterparts), I could book the flight with my UA number and receive my *G/1P benefits. To upgrade, I could then switch to my OnePass number, apply the miles, and then later switch back to my UA number for accrual.

In my case, I was modifying an existing reservation on UA to include this flight, and United's web site couldn't handle that, so I ended up calling web support to make it happen. Oh, and while I could pull up the flight by the CO flight number by clicking 'including Star Alliance' flights, it would only let me proceed if I had selected the UA flight number instead--the CO flight number would only bomb out. Based on my limited experience, I'm already envious of CO's web site and look forward to it replacing the UA equivalent.