Filing a request for UHT to be updated so that BlueprintNativeEvent and BlueprintImplementableEvent UFUNCTIONS() can support const TSubclassOf parameters.
This is easy to test - simply declare a BlueprintCallable, BlueprintNativeEvent UFunction in C++ and give it a TSubclassOf parameter. Code will compile so long as the parameter is non-const due to an error in the generated header files.
This also seems to apply to functions with Server, Client and NetMulticast as well.