@JustinG posted a tutorial (IIRC) but I can’t seem to find on how to create a dynamic icon that works like that. I don’t think Custom SVG Icon Sets is the one I’m remembering.
In the mean time, most of the icon sets for things representing levels (e.g. batteries) have three or four levels which you can define to correspond with any arbitrary values through expressions.
I don’t remember why I was told it doesn’t support that. I’ve seen your PR and will comment more there later this morning.