If the files haven't changed size, you don't need explicit support, just put the files in the right directory and let libtorrent treat them like damaged/incomplete downloads (it'll checksum the blocks and download the changes).
Dunno if it works for files of different sizes, though. It probably won't, since the blocks won't be in the same positions.
Dunno if it works for files of different sizes, though. It probably won't, since the blocks won't be in the same positions.