Aetheria Game Engine is an IF system that also implements RPG elements (combat mechanics, weapons, armor, magic, etc.) and allows real-time and multiplayer, so in principle it has the basic elements for a MUD. The system is multilingual and can produce games in English although, unfortunately, the documentation is only in Spanish at the moment. But since it’s online maybe you’d like to give it a go with Google Translate (from what I’ve seen, it makes a rather decent job).
Project site including downloads: code.google.com/p/aetheria/
Documentation: caad.es/aetheria/doc/