There's a list at http://www.riocar.org/modules.php?op=modload&name=Developer_Info&file=index&myfaq=yes&id_cat=9&categories=The+Empeg+Player+App#26 but it doesn't seem to be quite complete:

[menu]
sort_playlist=0

This setting makes the player sort playlist menus by play order (as seen in Emplode); the default is to always sort alphabetically, even though playback always occurs in play order. I think this was a tanstaafl wish.

[hush]
hushpercent=50

This controls how much "Hush" (on the Rio remote, or, if so configured, caused by cellphone mute) hushes the volume.

Peter