You are 100% correct I could use the shortcuts, and that's what I've been doing. I also agree this is probably a pretty specific use case, but I've been doing some CG painting and found that in my workflow I only really use each tool as described (click-drag for pencil, and single-click for fill) so this tool would allow me to stop having to swap back and forth.
I will admit, this is me being extra lazy because I even bought one of those mini-gaming keyboards where you can map any key to any value (or combination of values) which as been a game changer. It let me put the P and F key's right next to each other...but I'm always interested in ways of working smarter, not harder and I honestly don't know enough about plugins and the like to know if something like this could even be created.