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.

https://gist.github.com/DraTeots/e0c669608466470baa6c

Too late to edit, but there's also an unapplied pull request that seems to make usb passthrough work:

https://github.com/microsoft/WSL2-Linux-Kernel/pull/45

I have WSL1 alongside WSL2 for this. WSL1 works fine with my FTDI adapters to flash ESP8266 and all.