Hi Steve,
thanks for reporting this issue. Can you or Fabrice suggests what
change should be done in Org, if any? Also, perhaps another solution
can be found by adapting `org-file-apps'?
I am not sure that it is an issue that should be solved by org-mode.
It is more a problem of setting up WSL such that it meets well
some expectations. Basically, you need an equivalent of xdg-open
which redirects requests to the w32 system.
The suggestion I made to solve the issue is working for me, but as you see,
it is a crude hack: the paths need to be adapted to each individual setting.
If someone comes up with a cleaner, more general solution, solution,
it should be published somewhere as an example of how to setup things with WSL.
Fabrice