Is there a data dictionary for the rest API?
Accepted answer
The RTC Reportable REST API lists attributes that can be acquired. If you're looking for a more general API library, check out the CLM API landing page.
One other answer
It looks like the customAttribute property might do what you want. Check the com.ibm.team.workitem.WorkItem type in the RRAPI page. When you get that information there should be URIs for the custom attribute types you're interested in. You should then be able to do GETs on those URIs to get the actual values for the WI you started with.
- Jim