Is the Designer tab not visible on that blueprint?
PS: I edited my previous message with more detail, so check that out as well.
My game uses a similar pattern with base HUD in C++ so this should work. In my case I don’t have a need to make a message disappear after 3 seconds but even that should be possible by manipulating the FText you return from C++ to return a blank string after 3 seconds.
I just thought of one more option as well - whatever events are happening in C++ right now, have those classes propagate the events to their blueprint subclasses. From Blueprint you should be able to easily get a reference to your Blueprint HUD and then you can “push data” onto it or call the “Show/Hide” event directly from the other blueprint.