Extended BSP format with much higher vertex, clipnode etc. limits. Tools include TxQBSP2 (+ included skip and rotation fix), WVisBSP2 (threading support), and LightBSP2 (+ MH's coloured light and threading support). Engine support is in RMQe, documentation exists for engine coders.
The technical side at MH's blog:
http://mhquake.blogspot.com/2011/09/updates-for-13th-september-2011.html
Some comments from the design side by gb:
http://spawnhost.wordpress.com/2011/09/14/holy-shit-mh
I think there will be a release of the tools, engine and documentation, at MH's discretion, once everything has matured a bit, which can happen in a few weeks.
The stuff does not depend on RMQ the mod. It is also open source, of course.
I sincerely hope this will benefit people.