I was wondering if this was possible, or already created. There's a video on ToriTube of four players rendered in pov-ray, and I've seen the Player3 and Player4 head textures, but I haven't seen or heard anything else about it. I tried searching the forums for this topic, but got nothing relevant.

Is it currently possible to hack this into temporary use for singleplayer purposes, and/or will it be available as a future update? ...or, have I been completely missing something?

Yes, for now you can hack it into SP for fun. MP use will be sometime later though.

http://forum.toribash.com/showthread.php?t=18619 - 3/4 player match generator.

This has been asked enough times though.