As of right now, this is not easily achievable - also if you’d spawn a transition pawn, this would mess up with World Composition loading (as it’s pawn location based).
The simplest way would be to just add a full-screen UMG for the duration, though with BP there’s no way to check what that duration is (i.e. when the surrounding WC levels have been loaded).
For the latter, I can give you a code plugin though, that will monitor World Composition’s streaming activity, so you can remove the UMG in time.