I haven’t tried it, but yes, apparently so.
Sébastien Georget recently announced a personal initiative to allow Freeplane users to work together on a map without all the emailing back and forth, and spotting changes to copy to your own map. He calls it Freeplane-collaborative-tools.
Users work on their own local copies of the mindmap and periodically publish the modifications on a central server from where they can fetch other users contributions. The heavy lifting is done by your choice of versioning system: CVS, SVN or GIT, and this synchronizes the various copies of a map on request.
If you’re familiar with the Freeplane icon, I think you’ll like the Freeplane-collaborative-tools logo, it looks like a squadron in close formation:
Freeplane is open source, so the source code for Freeplane-collaborative-tools is open as well. There are introductory videos and documents here and they look pretty clear to me.
You might ask yourself ‘why not use a free web-based mapper?’ but for expert users of FreePlane, the fact that you can use all the hacks that this software offers to adapt it to your own needs and preferences can be a strong motivation.