In the interest of completeness, reporting back. Since defecting to UA, about half of my partner flights don't post automagically, and as I'm based overseas that is a fair few flights a year. 100% of the time I've used the form online it gives me the same result (the one I mentioned upthread): under status, it says "Denied. We are currently unable to process your request. Please try again later.". I always wait at least for 2 weeks after the last flight in the ticket was flown before submitting. While I appreciate the advice given upthread, I always submit the correct ticket, flight and seat numbers, along with the correct fare code, and I fill out the form relatively quickly so I'm not timing it out. Whatever it is, the form simply doesn't like me, and no matter how often I 'try later', it has the same result. Nevertheless, I still fill out the form each time, and of those, a majority of flights still end up posting within a week or two of me having filled out the form. No idea of course whether my flights just magically take ~4 weeks to post or whether the form is actually being submitted and the denial isn't accurate. That said, a non-trivial number of flights require me to contact United (I typically email 1K Voice) and ask for them to post manually, which does ultimately resolve the issue every time.

This is a rather annoying process, but in the grand scheme of things isn't the end of the world. To put it another way, I'd much rather go through this every few weeks than deal with my former airline and alliance.