Last modified: 2008-02-25 22:12:51 UTC
Could api.php?action=query&meta=siteinfo&siprop=namespaces contain information about namespace subpage settings (see http://www.mediawiki.org/wiki/Manual:%24wgNamespacesWithSubpages)? something like <ns id="1" subpage="true/false">Talk</ns>.
Added in r31278. Format is <ns id="1" subpages="">Talk</ns> for subpage namespaces, <ns id="8">MediaWiki</ns> for namespaces without subpages.