This sounds like a very good idea and probably should be the starting point.

Cool. I guess a plan is forming.

The meta model would have to be updated as well to express the constraints applying to the elements of a collection.

Sure

I think this extension could be done in a non-breaking compatible manner, only the Java 8 based provider would "populate" the new model elements though based on Java 8 type annotations, while the Java 7 based provider simply never would discover any collection element constraints.

Right, that was my thinking as well.