I don't know if the Zen Vision M has any UMS mode of its own, you'll have to ask elsewhere about that. As for enabling USB mass storage on other targets, I only know that PortalPlayer-based targets have no working USB in Rockbox (it's done with the OF.) Someone else will have to answer about the other targets.
For the status on the port, you can look
here. (That link is the first page, skip to the last for the most recent information...)