But I'm not exactly making an argument for information density, except that "it's kinda a weird thing and we probably don't know how to calculate it without ass

But I'm not exactly making an argument for information density, except that "it's kinda a weird thing and we probably don't know how to calculate it without assuming a whole lotta things". In my opinion, your extreme example actually supports my argument. Yes, you have to "share it ahead of time", but that's precisely what happened with the ascii encoding. It's just ascii is built into most systems by default and codified by a standards body, whereas our ad-hoc encoding is not. But does that mean ascii is inherently denser? I wouldn't use ascii to communicate with aliens because to understand it you require knowledge of the majority of the english language (again, english knowledge is shared ahead of time). In fact, the Voyager Golden Record has line drawings on the cover, and not a single character.