Perhaps a third tag SourceGuarantee, in addition to FullSource and ClosedSource, would be useful.
This could denote that the author has provided the full source to Epic, even though they do not wish to make it available to all buyers. It could be done with the agreement that should the author fail to live up to their commitments regarding support and updating to new engine versions (in a reasonable period of time), Epic would be free to then release the source to all buyers. This should allay fears about plugins dying due to the author abandoning them and the users being stuck with binary-only products.