What I want to do is do like the "When I receive []" hat block:
Where the data output is hidden by default, except I want to do it for a list of inputs in a custom block:
Sorry if it's a bit hard to understand, I tried my best. But, I might be able to give you more info if you ask.
Do you mean that you want to be able to have zero slots show by default rather than one slot? I agree; you should be able to specify minimum, default, and maximum number of slots. Jens has been resistant to that idea from the beginning of BYOB3, but now that we're making syntax the object of programming (i.e., metaprogramming) he's coming around on that sort of thing.