It should be possible to e.g. query the French wiki through the English wiki API, like
en.wikipedia.org/w/api.php?interwiki=fr&action=query&...
This is needed for scripts using XHR, which can't go off-domain. An implementation should preferably add a table mapping interwiki prefixes to api.php URLs (maybe use the existing interwiki table for this?), then kind of forward the request by acting as a proxy (send an HTTP request to fr.wikipedia.org and spitting the output back out to the user). 302 and friends probably (?) won't work here (XHR's gotta be smarter than that).
Version: unspecified
Severity: enhancement