We discussed this at the April 2015 EC meeting. From the minutes:
After a brief discussion we agreed that there is nothing today that prevents another standards organization from endorsing or normatively referencing a JSR. We also noted that JSRs can and do implement standards developed elsewhere. However, we agreed that a collaborative development of standards (where the JCP and another standards organization work together) is unlikely to be possible due to differing IPR policies.
We therefore agreed that there is probably not much to do here but to warn (as we suggest in the IP-Flow document) that we should not introduce any language into the JSPA or the Process Document that would make it difficult or impossible for another standards organization to normatively reference a JSR.
I will leave this open so that we can do a final review to make sure that we aren't doing anything to impede normative references to our JSRs.