Does anyone know if it's possible to rename the inventory tab to say something else or even to redesign the interface altogether with pngs?

I know there are some tutorials within the documentation regarding this (even though not specifically to that pane, but haven't tried that myself yet. You'll need to know some CSS and maybe JavaScript to do that.

As far as I can tell, QuestJS / Quest 6 will be capable of that as well (search forum for info on that), but it is not yet finished yet.