I looked into a more general way to patch your config in,
https://gerrit.wikimedia.org/r/#/c/194030/
but, low and behold, there’s already a method to do it.
Set fetchConfig to false in your localsetting.js.
Then, assuming cruiserswiki as your prefix,
(as in parsoidConfig.setInterwiki( 'cruiserswiki', 'http://www.cruiserswiki.org/api.php' );)
you’d add a cruiserswiki.json to /lib/baseconf/ and that should load it.
https://github.com/wikimedia/parsoid/blob/master/lib/mediawiki.parser.environment.js#L306-L316
To get that cruiserswiki.json,
curl "http://www.cruiserswiki.org/api.php?meta=siteinfo&format=json&action=query&siprop=namespaces|namespacealiases|magicwords|functionhooks|extensiontags|general|interwikimap|languages|protocols" > cruiserswiki.json
Then you can add,
"extensiontags": [
"<pre>",
"<nowiki>",
"<gallery>”,
“<imagemap>"
],
for the <imagemap> as requested in,
https://lists.wikimedia.org/pipermail/wikitext-l/2015-March/000931.html
Unfortunately, as cscott suggested, this doesn’t help the current
mediatype issue. Sorry.