The RTC work item schema provides queries based upon the type that the custom attribute actually is i.e. short/medium/long string extension, int extension, enumeration, etc.  Also note that there have been several improvements made to the work item schema from RTC 3.0.1 to present.  For example, take a look at:

https://jazz.net/jazz/web/projects/Rational%20Team%20Concert#action=com.ibm.team.workitem.viewWorkItem&id=232011

In short, you must know the type of custom attribute as defined in your project.  Then there should be  corresponding query to that type in the work item schema as seen in an RPE template.  For each custom attribute type there are name/value pairs of queries, so you first search for the name of the custom attribute that you're looking for under the type node in the schema, then print its value via the value query.  I'll also note that native filters really improve generation performance and make your template work easier when you're trying to print a specific custom attribute.