I have been using Unison to sync directories on my N900 & a external hard drive on an Ubuntu desktop.

Rather than install Unison on the N900, Unison is installed only on the Ubuntu machine, & the N900 is connected in Mass Storage mode via USB. The N900 directory to be synced is discovered as the local machine's /media/1xx1-xxx1/whatever directory, & Unison easily works.

Is it possible when I connect the Ubuntu machine to the N990 over wifi ssh/sftp to find or write that path as a local path on the Ubuntu machine?