I had the idea based on my Mighty Mule driveway announcer modification. I went looking and, as usual, somebody else had already done it.
Works! I used the EcoLink DWZWAVE2.5 contact sensor.
-update-
Because I have a 'standard' size mailbox which is routinely stuffed/wedged with Amazon boxes, I elected to remotely locate the transmitter on the post and use a small magnetic switch in the jamb. Like so
