All I know is there should be a way to do a SELECT on table wiki_page WHERE the page_namespaces are in, or not in, a list we give, and output that in export format. Seems like a very direct path. Sorry I have not yet looked at any API code.
xexport&ns=all ns=1,2,3 xns=4,5 ns=all ns=-1,-2... ¤t_only(default)... ns=1|2|-1 ... I remember there are negative namespaces, so must think twice...
That way the API would then be able to do general exports, better than Special:Export and dumpBackup.php (except for a few of the latter's additional options...)
Then we can have a button on our wikis: "Click here to export a backup of all the pages on this wiki before the cops arrive and confiscate it!
That way you can let all the police radio frequency tables of http://radioscanningtw.jidanni.org/ live on, at least in your own home, while the former maintainer is serving out his sentence for breaking draconian national security laws, if there one day happens to be such laws.
Sure, we exclude user data, because that's confidential."
And the backup can be up to the minute (not just to a link to a .xml file made by a nightly cronjob). And it can be done all via one single API HTTP GET with no need for some homebrew PHP CGI script.