I think this is a good proposal (thanks). I don't think it would break backwards compatibility (i.e. existing application code wouldn't need to be changed).
However now JMS 2.0 has reached the public draft stage we need to avoid adding new features if at all possible and as this feature is unrelated to any of the changes in JMS 2.0, and can be added to the next version without introducing any compatibility problems, I propose we defer adding it to the API until JMS 2.1.
(The exception to this is javax.jms.Message#getPropertyNames(), which is new in JMS 2.0 and we are separately considering changing to return a Set <String>).