It's configurable per cache. If different apps share the same cache (e.g. web session replication), then, no, it's not configurable per app.
As for the process of picking buddies, I'd say its more extensible than configurable. That is, you specify your buddy locator class, so you could write your own implementation of the BuddyLocator interface that gives you custom behavior you want. (And, of course, if it's generically useful, please contribute it back.)