Since OpenMRS 1.9.2 it is possible to use JPA annotations in modules. See more at:
Univ Call: http://connect.iu.edu/p6erdt413mz/
https://github.com/rapilabs/openmrs-jpa-annotation-test
https://github.com/rapilabs/openmrs-jpa-annotation-testhttps://github.com/rkorytkowski/openmrs-core/commit/416ca5addf1c50ba65c6982fc55aaceb142d2e44