I have an idea - could we implement a cast bar for Hunters' Auto Shot? That way, I could reduce the number of add-ons I need to install.

Hey,
probably not; swingtimer etc is too specialized, it has to be good to be useful, and atm I cant afford to sink that much time in it, especially if its different in every supported version (classic/SoD, Wrath, Cata, soon MoP).
Someone asked about aimed shot, and thats kind of bugged as its not firing the default casting events in the API. You can detect it, but its a lot of work.
I recommend using something like https://www.curseforge.com/wow/addons/weaponswingtimer-sixxfix (seems to work, but is kind of ugly I think, maybe I will make a custom skin for a that :D )