Thanks both. Eschwartz, I think the directory-watch feature will not suit my usage as metadata needs to be manually added with uploads. So maybe I have to wait for v3, and find a server where I can install calibre (so far I'm hosted by gandi, and I don't think I can have calibre there).
Or maybe an alternative technical solution for me might be to use the "send to folder" feature to a dropbox-like cloud folder -- I can actually install ownCloud on the server, and have all my collaborators install the ownCloud desktop client. They can then use this server folder as a device and upload books there with their metadata.
Maybe I could in addition have COPS running on the server for a convenient mobile access ? (read-only access, that'd remove the need of the ownCloud client for this)
Anyone can comment? I'll give it a try and report if no one sees a big fault in this plan.