I tried that but it does not work, neither with the I interface or the U interface.
UPROPERTY(EditAnywhere, meta = (MustImplement = "USelectable"))
TSubclassOf<UObject> Selected;
UPROPERTY(EditAnywhere, meta = (MustImplement = "ISelectable"))
TSubclassOf<UObject> Selected;
Dropdown list contains every UObject in both cases.