I know a podcast that did a few movie commentaries that were simply live. Obviously, not near Rifftrax level of consistent funny, lots of dead moments, but also some great comments and reactions and forged runnign gags.
Seems to me that if you can just get together and have a dry run that gets recorded, you'll not only perhaps figure out potential material on the spot (and of course the five mnute later 'that was the joke' moments would occur) but it might give you the enthusiasm to actually do a script and make a tighter second run to submit to Rifftrax.
At the very least, the raw mp3 would be a sop to the RPPR community and mean you don't have to do a proper show if you posted that!