So, quick(ish) update.
Getting closer to first actual migration - and progress is certainly more than "inching" forward.
By yesterday A.M., had whipped up bit of code aimed towards web-scraping all member preferences (except their password). Could ask DreamHost for data dump of that, but the work involved to get them to actually do that times the probability they wouldn't do it anyway or would screw it up - I much prefer going with the sure-fire web scrape - and yes, that'll mean new passwords for members, but if that's about all they have to deal with, I figure that's pretty good. And I much prefer to not have well over 800 members all have their list preference settings all get squashed to default or the same across all on per-list basis - that wouldn't be very friendly. Anyway, code isn't completed, but got it yesterday A.M. through point where it gets all the member's email addresses, and has the URLs to each member's individual preference page - which has all their settings except can't extract existing password. So, bit more coding to grab/parse that, and fairly close to migration ready.
The other bit is *loading* preferences. Have looked over Mailman DB stuff - it uses Python pickle format. I don't (yet) grok Python, but looks highly doable for me to work that out. Looks like all the per-user settings are basically some strings (e.g. optional full name if entered), and a binary mapping (e.g. 2^1 bit is toggle for option 1, 2^2 bit is toggle for option 2, etc. (at least approximately - might be some with more than 1 bit for options with more than binary selection). Anyway, shouldn't be too hard to work out those various bits and test (probably do some add/drop of a scratch/test list for testing some of that out - also at same time, tweak Mailman config a bit so any newly created list comes into existence with desired defaults).
Hmmm, I should'a written the web scrape stuff to pull roster years ago ... would'a saved me about 5 to 10 or so minutes a month (okay, still would'a been fairly long ROI, but would've paid off by now).