I think most of the work here would be about getting the UI right. Obtaining the patterns that make up the song, easy. Length of each pattern, easy.
The “hard part” is to represent this on the screen in a way that adapts to whatever size your tool might have.
Actually, I have been dabbling a bit with this. My (slightly delayed 
)update to the selection-shaper tool has a helper function which can be used for representing the relative size of a list of numbers.
So you give it a horizontal size and hand over all the pattern lengths. Voilá, it will then give you the size your buttons/whatever would need to have.
It’s used for building stuff like the keyzone and slice marker visualization in this screenshot:
https://forum.renoise.com/t/new-tool-3-0-selection-shaper/41240
You’re welcome to dive into the code, of course. Something to copy-paste, experiment with… 
The helper function: (“distribute sizes”)
UI widgetimplemention (“vButtonStrip”)