GeSHi Sourceforge group redirects MediaWiki-related issues to MediaWiki. MediaWiki redirects requests & features back to SourceForge.
I modified my install to parse PHP blocks allowing to highlight HTML/JS/CSS as well.
As you know, the MediaWiki extension lives in a directory named SyntaxHighlight_GeSHi, and contains a subdirectory geshi. If your patch is to the files inside geshi, submit the patch to the GeSHi Sourceforge group. If your patch is to the files not in geshi, submit it to MediaWiki's bugzilla.
If your patch is to both, hopefully the part of the patch to files inside geshi is independently useful. If so, submit that part to the Sourceforge group and the other part to MediaWiki. If the patch to the geshi files is not independently useful, you may have problems convincing either group to take it.