There should be an XML or other kind of feed that can facilitate transfer of diff data for each revision in real time, for import into another wiki running MediaWiki. It evidently needs to be less expensive than the present OAI-PMH functionality, which had to be restricted from public access due to cost. But it also needs to be in a format that is more suitable than RSS for import to a wiki.
There will need to be a client side and a server side. The extension on the client side should not interfere with the ability for editing to take place on the client wiki, although revisions being imported can supersede user-contributed revisions, in a similar manner as Special:Import. Two wikis with partially overlapping content can thus mutually collaborate on a set of articles if the wikis are configured as both clients and servers of each other.