Suppose I had a framebuilder weld disc brake tabs on the existing fork (or another steel threaded 1" road fork of the same height and rake, if I don't want to mess up the bike's original fork).
I wouldn't,those legs look a touch spindly for that.
I have read that disc brake forks are built stiffer than rim brake forks, but how unsatisfactory would this actually be?
Depends on the fork. Disc forks are beefier,but CF dampens vibes better than steel,but it still comes down to the specific fork's design.
Honestly,I'd pick a different bike to do this with. Between the 1" steerer and threading,you're looking at having to swap out too many parts to modernize it to take a disc. I'd hunt around for a used Schwinn DBX or,if you can tolerate an alloy fork,look into a Novara Zealo on clearance. Or if this is just going to be a bad weather beater,look into one of BikesDirect's disc cross bikes.