Well, I went by VeloCult and talked to them about my desires and budget. Their recommendation:
- have a local frame builder make me a disc fork with height and rake identical to the existing fork, 1" threaded.
- get a TRP Spyre mechanical disc brake
- build a new front wheel and voila
So I'm going to call some local frame builders and see what this might entail.
Sift through the selection of used forks at Citybikes. See if there's something with the correct size steerer and heavy-ish blades (don't ask me how you would figure this out). Then have a framebuilder add a mount for a disk brake. It's got to be cheaper than a 100% custom fork.
Suggestion from out in left field: Bacchetta (the recumbent people) offer a steel fork with a 1" steerer and a disk mount:
http://shop.bacchettabikes.com/parts...w-1-steer.html
It nominally fits a 26" wheel, but they say it's got enough room for a 700C. You'd still need to do a threadless conversion, but it might turn out cheaper.