IIRC, I've been using PuTTY's agent (Pageant) for this kind of purposes (and it's recognized by FZ, IIRC) whenever I've had to suffer through a session of debugging on Windows.
It's mildly annoying because you need to convert an OpenSSH key to PuTTy's format, but, eh.
It's also completely orthogonal to the original question, as that's always in OpenSSH format anyway
.
Putty's keygen/convert prints it out nicely, though, if memory serves.