Thanks for the suggestions, y'all. The shoot went pretty well and I'm eager to get developin'.
I have done push processing in ID-11 before, but mostly with Arista Premium and Kentmere 400 which have done well up to 1600, and Delta 3200 which like I said in my first post got good results exposing for 6400 and developing for 12500. My one attempt at a 3-stop push of HP5+ in ID-11 didn't go well, but I'm pretty sure that was because the exposures were bad in the first place, rather than bad development.
From the sounds of the feedback you've given and Ilford's own data, I'll grab some Microphen when I'm in Toronto at the end of the month and give that a whirl. I am curious to see how ID-11 performs with these films, because it is my go-to developer, but I'll save those tests for rolls I don't care about as much.