So, as of now, the "Show additional confirmation numbers" option on the UA reservation website appears not to be available any longer.

I tried to access my UA award reservation with LX on the LX website using the UA PNR, but had no success. I called UA to get the Amadeus PNR (a fairly quick and efficient call, although I would have preferred to find the information online still). But the LX website apparently still wouldn't let me access my record using the LX PNR. I ended up calling LX to make my C class seat changes.

I subsequently discovered that the problem source, at least in my case, was that I had entered my real first name, rather than the amalgamation of first name and middle initial that UA adopts in its booking record. [In my case, the UA version gives me a sex change, so I don't think to use it.] The LX system isn't forgiving in that regard: you need to use the precise UA record "first name." With that entered, the LX system currently will find the booking using either the UA or LX PNR. And interestingly, when the booking has been found, the LX website shows only the original UA PNR, with no mention anywhere of the Amadeus PNR.