I think there are two different types of tasker notification icons.
One is the type that a user/programmer generates from a tasker notify action (in a tasker program). It can be made "permanent" until a cancel action is sent from tasker.
You can control that icon via a tasker program (and build a profile to update it every time battery level changes.... and one of choices is to update it based on a filenmame... which you could build as a string variable based on battery level).
The other type is the permanent one. I think this is what OP was talking about since he mentioned he was stuck with it.
I think most people go to preferences and adjust so it is not displayed.
Preferences / Monitor / Notification Icon / Unchecked.
Doesn't that work for you?
If for some reason that doesn't work... I don't think it can be adjusted programmatically... it is only accessible via prefernces menu and not by tasker command. I guess there is a small possibility you could point it to a filename and then overwrite the graphic file stored in that same filename, but I'm not sure that would work.
You'd need a profile to update.
When battery level changes: build the filename based on battery level, then set
The filename would be the variable.
The task would