Ok, my restore did work in the end, but it never gives me any indication of when it's done, so I just have to guess and hope for the best then cancel out.

This happens with both optfs and rootfs.

Summarise:

Backing up:
Both optfs and rootfs, individually or together:
No error messages, however just hangs after a certain size but creates a working tar file.

Restoring:
Both optfs and rootfs, individually or together:
No error messages, no indication of progress (Size: N/A). However does seem to restore, just have to leave it for long enough then cancel out.

RootFS took 400+ seconds to back up, 100-200 to restore.

Edit: this happens on 2 N900s on PR1.3, one has very few apps installed and is not too different from stock, other has lots and lots of apps installed including custom scripts etc.