Shoot!
So my best option would be to make my own GUI element from scratch that "looks"/mimicks the on hover inspector to add my own elements.

Is it possible to show/hide a GUI element on the player hovering over a specific prototype?

On the panel that shows details about an item (like liquid amount in a pipe), I'd like to add GUI elements/info for some of my custom prototypes.

I cannot find a play to really start on that, though I see LuaGuiElement, and I could probably iterate down the GUI tree to find the parent I want, I'd ...