http://www.purifresh.com/erv.html

https://swervair.com/

A couple examples I see on Google. I'm not advocating for any of these, because I have no idea if they are any good, but I see no technical reason an ERV couldn't work as a window unit. Maybe it's an underserved market and someone should make a business out of that.

A much more DIY example that's probably closer to what you were talking about with "hacking together" a solution: https://www.mychemicalfreehouse.net/2023/10/window-mounted-p...

Thank you! You're right that's the sort of "hacked together" solution that looks cool but beyond my abilities, and I appreciate the first two links.