I figure the concept from http://tracker.firebirdsql.org/browse/CORE-2666 should be applicable to nbackups and should be implementable in basically the same way. Restoring should also be possible but I guess it will be a bit harder than gbak since you could potentially have several files have to differentiate between them somehow.
I imagine it would be somewhat heavy on the network for lvl0, but other levels are notable smaller.