I had to fix textures in both my Ocean & Sky and Waterfall painting sets, and while I made note on their individual download pages that people should re-download, is there any other way to alert people? On nexusmods.com, there is both versioning as well as a download manager which alerts people to changes every time they open the download manager or revisit the site to check out a given game's content, but I don't think MTS has anything of the sort as far as broadcasting alerts, unless I've missed it.

MTS has the "Latest Updated" feature, which is in built into the download browser and the profile pages. Additionally, the My Downloads link has an "Up to Date?" option where people can sort the list by when they last downloaded vs when the mod was updated.

I see them now, thanks. Is there a proactive way to alert people who've already downloaded?

Unfortunately not, unless they're subscribed to your download thread Usually what I do is just adding a big banner saying 'new update!' and most of the time when people find issues in their game, they'll look up the download again and notice said banner (or text!)