I can use the same mplayer command and redirect stdout to a wav file, scan and then play it via FireFly!
I think the (imho) righter way to do that is to make a dummy .url file and point it to a web service that transcodes the file.
Like, make a cgi on a webserver that does the transcoding, and then point the url file to that.
It’s probably possible to abuse firefly to do it, but it’s really fundamentally not set up for streaming. Just to serve files.