I think MS is trying to both support and not-support WSL, in the sense of building it out enough to support their internal agenda, but not where complex SLAs are needed.
So, COM port access... I don't see that being hard; I envision it as boring API-wiring, since RS232 has been standard since before Windows and Linux existed, so it's likely the code's generally similar.
But in terms of high-level support, RS232 is primarily industrial-class, with a long tail of developers and workstation uses trailing after it. Industrial immediately means complex SLAs and - nooooo - industry certifications and the like. I don't think MS wants to invest that much into WSL.
With this said, a bit of poking around found https://blogs.msdn.microsoft.com/wsl/2017/04/14/serial-suppo..., from last year. Are you on Insider Build #16176 or later?