gpgrt_poll can support BACKEND_FD.
Initial patch is: D541: w32: Support poll with FD backend
I don't know how runtime (of mingw) is thread-safe, but if it is, it should work well.
I do not think that we should put any more support for FDs into gpgrt. The goal is to move entirely to the Win32 API.
The original intention was to fix t-poll failure on Windows.
It was fixed in different way in rE858bcd4343ac: tests,w32: Use CreatePipe and es_sysopen..
So, closing this ticket.