Is there a way to queue via command line files to Mplayer's playlist? I'm trying to make right click entries for some filetypes to do just that, but I had no luck so far. Any ideas?

While loading files can be done by Rexx in both GUI and CLI mode, appending is only doable in CLI mode, it would seem. I'llfix that for next version.

Anyway, this is the way to do it (and it will work as intended in GUI mode in next version).
rx "address MPLAYER.1; 'raw loadfile <file> 1'" (1 is the flag to append instead of playing)

Thx Fab! :)