Maybe an actual chemist will chime in, but I couldn't find any sources about purifying or recycling IPA, at least none that you could do at home. At a guess, maybe a reverse osmosis system without the finest membrane? Like you said, the water and IPA molecules should be smaller than the dyes and resins.
Maybe also consider washing with the dirty IPA, and just giving a rinse at the end with the clean IPA in order to conserve. Ultimately it sounds like everything I've found is material you're already familiar with, unfortunately.