When I talked with the guys over at HA who manage the MQTT side of things this was one of the exact reasons they said there is no MQTT autodiscovery for buttons.
IIRC, when i was looking to work with buttons originally (waaaaay back in one of the early alphas) the closest that I came up with for a working solution was to pass to the MQTT topic a multi-value being "{button}:{state}" and then on the HA side, parsed that to determine which button and what (pushed, held, etc)
Regardless, it wasn't worth it in the end at least for what I was trying to do.
