It seems that it's impossible to create a Universal joint.
When trying the XML parser says :
[xmlreaders.cpp:522] xmlSAXUserParseFile: error parsing file: openrave (Failed): KinBody::Joint::GetValues: unknown joint type 0x80000001
(N.B. : Tried on Ubuntu 11.04, Openrave 0.3.2 and OpenRave 0.4.0)
The error comes in fact from the file kinbody.cpp, in the function void KinBody::Joint::GetValues(vector<dReal>& pValues, bool bAppend) const. In the switch .. case function only JointHinge2 and JointSpherical are tested.
created a ticket to track this item: