Most likely, yes. As long as they have the correct format, and correct layout, it'd be as simple as replacing the files. Since the (DXT) graphics are deterministic in size (always compressed 1:4), if they keep the same layout, they keep the same file size and the file offsets in the rest of the file don't change, so there shouldn't even be a problem if there's hacky hardcoded direct offsets in the .exe (which is unlikely these days anyway).
Not something I'm looking into for a while, but I'll probably release some specs when I get back from vacation in a few weeks (if BG or someone else hasn't already).
It's even likely, as in GrimE, that just placing the new art in the proper subfolder, with the proper name, in the correct format, might just mean it's loaded into the game. Since that's easier (and very unlikely not to be implemented) while developing. If it's not turned off on release.