Supporting move to "All wikis in a given language" is probably a pretty tall order since this was made as a one to one tool (and how would you merge multiple divergent edit histories?). But if we could get, say, FileExporter on Commons and FileImporter on English Wikisource, it would make it a lot easier when we have to repatriate books that are about to be deleted on Commons. That should be at least within the realm of a reasonable short term goal; and with that in place it may be possible to create user scripts or other external tools that can semi-automate the "All wikis in a given language" bit.

For reference, in English Wikisource it is not uncommon to have files that have roundtripped multiple times between enws and Commons due to an unclear copyright situation, Commons or local policy changes, or simply the passage of time that means copyrights have since expired (this happens somewhat more frequently with books than with other kinds of media). Just yesterday I was moving files to Commons that had been previously moved from Commons but whose non-US copyright has now expired: the ability to roundtrip this without loss of edit history etc. would be very useful.