Interesting, but I don't think the miking will be that sophisticated.

Some back-and-forth with the manufacturer confirms the approach I thought would be required.  In a nut shell, mic the piano, mix the mic and audio out from the electronic accompaniment together with a little mixer and then send that to the DA equipment.

Their recommendation for a permanent installation like this is the Barcus Berry piano mic which is something I've used in the past.  It sticks right onto the sound board of the piano.  My experience is that it's not the best sounding solution, but they made a very valid point that the Barcus Berry won't pick up ambient sounds like a traditional boundary mic.  This minimizes the concern of privacy and keeps the piano mic from picking up people talking near the piano and broadcasting it through the house which isn't something I'd thought about.