You can do that now, but it isn't officially supported
just use the -position tag when you launch it.
the resolution of your primary monitor is used as a base
for a display above your main display make the y value negative the secondary displays res
(-position 0,-640,480,640 or something to the effect)
The "-listen" tag has been around since the first release. It allows j5 to run constantly, and it merely checks a text file called "clo.txt" in it's base directory every few seconds for changes. Combine the two, being sure to pass the listen command when you re-write the text and you have an app that stays on the secondary monitor and updates itself whenever a new games is launched.