The LUA autofire plugin does work. I have no emus installed on that computer here to take a look so maybe I forgot what the right setting are but it is a thing (was added to MAME like a few builds ag, I've used it)
EDIT: maybe they've changed how to use it, dunno, i'll have a look tomorrow. It's very new so they've made changes to it like every build since its introduction.
EDIT2: yeah it works as I described. Maybe you meant you couldn't figure how to make it work? me neither the first time, it's pretty confusing, and of course there's no documentation on it.
To begin I think (to be confirmed tho) that the AF-enabled button (hotkey) can't be same as the default one, this is to allow having both the normal and its AF-enabled duplicate on the same controller if desired. So it's up to you to arrange your inputs for that, but you can assign a button away like on the keyboard if you don't want to keep both on the same controller.
Then the frame press/release arrangement is pretty self-explanatory.
Settings are saved in .cfg's