Hi Gunnar,
Well, I think it is not worth to implement if it raises no portability.
The other solution to the problem is just to create custom composite constraint annotations based on standard ones.
As an example:

<bean class="com.my.User" ignore-annotations="true">
  <field name="email">
     <constraint annotation="com.my.constraint.EmailPattern"/>
  </field>
  .....
</bean>