http://www.doomworld.com/vb/showthread.php?s=&postid=546372#post546372
eternity -file av.wad -warp 01 -skill 5 -record an01-xxx
records a demo temporarily named an01-xxx.lmp using Eternity, on Nightmare skill starting on map01 of Alien Vendetta.

http://forums.newdoom.com/archive/index.php/t-22807.html
-file void.wad -playdemo clvoid52
plays a named demo clvoid52.lmp using a required named wad file void.wad.