I reckon this should be passed on to the user. At the end of the day when we talk about 'users' in this context, it is not end-users who write Java applications, but rather developers who implement frameworks such as Tomcat session clustering (a.k.a. Brian :) )
A separate method call will just add clutter. If PojoCache needs to be aware of a data gravitation (so it can gravitate internal space as well) then it should sniff for the option being present in the Invocation Context.