Diafine B is basically just an alkali solution, so I think there's every reason to expect it would last until the end of the world. The A solution with the developing agents does fail eventually, but there seems to be a fair amount of variation. Mine has generally lasted on the order of a year, but I've seen people say they were still using a batch successfully that was 5+ years old. I think basically everyone keeps it in bottles that are at least close to airtight; your "semi-open-air" proposal might be the first of its kind.

Personally, I don't find that it causes dull highlights in a visually obtrusive way when used as a contrast-taming developer in the way you're suggesting. Tri-X in Diafine does have a very distinctive look that either is or isn't what you want, but it sounds like your main target is the aerial films anyway. I think it's worth pursuing, but you should definitely do a few test rolls first to see how it works.

You said the film is "old-style", but is it in-date? In my experience, Diafine can be a problem with outdated film, because it doesn't control fog worth the proverbial hill of beans.

-NT