It's a little clunky, but you could use a Windows RFC2217 remote serial server with socat on the Linux side to make a virtual serial port tied to a real one. I've done this (albeit not with WSL), and it did work.
Too late to edit, but there's also an unapplied pull request that seems to make usb passthrough work:
Other that this I love Windows 10 + WSL2 as a dev environment.