Energy Tanks and Invul Frames

[Downloader]

There was a script I was using for an item that acted like a fairy in a bottle. Not sure if that will be of help.

How do I use the subscreen to display full or empty health tanks?

Note that with scripting you can draw anything in the passive subscreen area by setting the Y, for example, in a drawtile or drawinteger function, to a negative number that is less than zero but greater than around -64 or so.
To draw that type of thing to the active subscreen, you might need to write code that adds and takes away items based on whether the Energy Tank is full or not, and overlap their space on the subscreen so that the item changes there depending on if the tank is full or not.