Information on JDO dynamic fetch groups at http://www.datanucleus.org/products/datanucleus/jdo/fetchgroup.html#dynamic
My thought is that Isis would spin through each of the classes, and look for those that have any collections annotated EAGERLY. It would then programmatically define a JDO fetch group for each.
When the object is displayed, Isis would select ONE of these fetch groups and use it to avoid some of the N+1 issues.
NB: it probably would need to select just one of them, otherwise the resulting query submitted by DataNucleus would be of the form
JOIN child_entity1 ON ...
JOIN child_entity2 ON ...
ie, a rather large amount of data...