OP here, finally following up on this. I've spent a lot of time talking to people, all of whom were very nice and generous with their time, but I seem to be at a dead end.
ProAc confirmed that the original woofer was the SEAS HO803-08, but that they don't have any spare parts for this. I've spoken with new and old SEAS dealers who also don't have a replacement (indeed, though ProAc was very specific about the model number, I can't find any evidence for it anywhere).
I spoke with the Sound Organization, the US distributor, who also don't have any replacements. They said that simply dropping in a similar-size woofer would be a bad idea, due to issues with how specifically the originals were designed. They recommended a few different repair or re-coning shops, none of whom were able to help. Madisound were unable to help, and I spoke with a few dealers (including the NJ and the Toronto ones) who also didn't have any ideas.
I think I've done all I can do, and as this particular line of inquiry is unpleasantly stressful for me, I think it's time to throw in the towel. I'd welcome advice about how to get rid of these—perhaps someone is more patient than I am, or needs just one perfect speaker, or needs parts.... Getting some money would be nice, or I could just give them away.
Thanks for sticking with this!