I have finally gotten back to this project and created a working sketch that communicates with my Vera controller. I have this set up as a 3 button on/off (6 switches on the board) and have that working successfully with Vera. I also have my Vera sending temperature data back to the OLED display. It alternates between the outside temp and room temp for my master bedroom where the switch is located. I have posted the Arduino sketch under source code here.
In the BACnet protocol, the messages are not sent directly, but pushed into an internal state machine that remembers (some) of the previously messages sent (i.e. memorize(_msg); send(_msg); ).
The state machine also peeks incoming messages and can trigger a message re-send in case of a missed ACK.
the difference with MySensors is that all the messages are numbered, otherwise you would not know which message of the last N sent previously was ACKed,
but well.. ACKing the last similar message might be good enough.
Anyway @SiLeX : if you want to try, this is not that complicated, (if you don't take into account the STREAMs) it will just costs about (1 status byte + 1 timestamp + 1 frame size) x N ; for N frames memorized.