I can probably come up with half a dozen heterochromatic characters in AVNs. I've met way more people in my life than the number of AVNs I've played and haven't met a single one. (Have met an albino once though.)
Here are a couple more,
Annie from Last Call has an important cameo with Mallory in C.S.
Veronica, from Artemis
There are a few more I remember but not their name or game, I'd have to dig around to find them. I just found it an odd fad to occur in the genre, but am surprised no one else noticed here.
Edit: Jeez, been playing WVM and just noticed a character was one in this game too (the teacher, Veronica), it took a while to notice though because she is brown/hazel and I guess I was too busy reading the text to notice it in closeups before.
Also in WVM Veronica's sister, with the pink hair, was blue/green.
Holly in Corrupted Love.
(this is for science! and posterity! Not just coin points.)
Just checked out "My Dorm" and look, the gay best friend's sister Simone is brown/blue.
Edit: Nope I was wrong, the lighting was funky or something in the render where I saw it. A closeup a few screens later show them as the same color clearly.
Blanche in Color Of My Sound, though I'm not sure she counts because it isn't clear she is heterochromatic. Her backstory is that she was kinda re-assembled after massive traumatic injuries sustained in an attack, so are those actually her original eyes? Not that it matters, its just an in-story explanation for why yet another AVN character has different colored eyes.
I have a sister that got hit with a stick in her eye as a kid and made her Heterochromia so if someone don't know it is when your eyes are different colors.
WVM gets another, Iris, a woman from the MC's past. Is she related to the heterochromatic sisters? I don't think we know yet (but probably, an aunt or something.)
Um, I made a post a couple days ago that isn't here anymore. I thought it was kinda funny but now I wonder if I broke a rule and a mod deleted it? I just don't know what that rule was and I don't have any notification of such a correction. We're obviously talking about various games here, someone else included a screenshot from one of them already (and my screenshot was just the eyes, not even the whole character)... My jokes were lame but I don't see how they broke any rules. I guess I'll just go super vague on the details in case there was a spoiler in there that offended someone or something like that.
Anyway, the one I posted about was an unnamed single scene character in UC8, the secretary at the youngest girl's school when she was being withdrawn before moving.