Thanks for working on this!
I have no strong opinion on the general approach. Making the EPT trigger major GCs after a while (i.e. when the EPT has grown significantly without a major GC having happened due to other triggers) might also be acceptable, and might be (much) simpler.
Speaking of simplification, if you go with the suggested route of adding a Minor GC for the EPT, it might be fine to ignore the Scavenger and only support the MinorMS (which, IIUC, is intended to become the only supported young-generation GC at some point).
+Michael for a GC perspective.