Does the Rockbox firmware work with MC?
It does, but it actually doesn't have to. One really nice feature of the Rockbox firmware is that you can always boot the iPod to the original Apple firmware by rebooting the iPod (Menu + Select), then holding down the Menu button until the screen says "Loading original firmware." Once you do this, the iPod is effectively a normal iPod. When it's in this state, the Rockbox software is just a bunch of files on the iPod that are ignored, so any software that interfaces with the iPod "sees" it as a normal iPod.
If the iPod is booted to the Rockbox firmware, MC still sees it as an iPod and transfers syncs to it, but there are a few catches. First, in order to have Rockbox see the new songs, you'll need to go into the Rockbox settings and either "update" the database or "re-initialize" it. It needs to do this since MC does not (at this time) automatically do this for you. Second, the transfer speeds "might" be effected if the iPod is booted with the Rockbox firmware when MC syncs it. This was an issue a while back -- transfer speeds were a little over half of what they were when doing the same sync while the iPod was booted to the original firmware. I don't know if this is still an issue, however, since I always boot the iPod to the original firmware before doing a sync. I'll have to test this again one of these days to see if this is still an issue.
It would be GREAT if MC could take care of the Rockbox database the same way it takes care of the original firmware database, but this may be a bit much to ask since Rockbox development is not "official." In the mean time, you just have to remember to run a Rockbox "database update" after an MC sync.
I guess the short answer to your question would therefore be: Yes.
Larry