I have seen that you can clean used IPA from resin by putting it in the sun a while, making the resin dissolved in the IPA cure. Could I do the same thing just using the cure-mode in with the Elegoo Mercury wash-and-cure-station?
Based on the frequencies of most UV cure resins, your suggestion has merit. Other factors come into consideration, however.
These illumination devices have a limited life span. Running the UV diodes for the purpose of curing out the resin from the IPA will "use up" the portion of time otherwise useful for curing models.
Additionally, the UV light which would otherwise be "applied" to the surface of a printed model now has to pass through the IPA. While the IPA is perfectly transparent to the 405 nm used to cure resins, there could be attenuation and refraction by impurities in the liquid.
Sunlight is mostly free, broad-spectrum, and (on a sunny day) far more powerful than most affordable UV illumination systems.
In a pinch, sure, use the curing feature, but the sun is your cheap friend.