I added a umask keyword parameter to both fcgi.WSGIServer and fcgi_fork.WSGIServer. If given and binding to a UNIX socket, the umask will be changed right before the socket is bound. Afterwards, the umask will be restored.
This should allow one to change the file permissions of the socket without altering the umask of the entire application. (If you need it to be, say, group-writable or world-writable.)