Hi Mike,
Looks like the link you posted has the sensor end of things pretty well covered. The problem would be connecting both an xbee and the OSBo board to a BBB. A quick internet search brought up some links to an xbee cape for the BBB but it looks like that would preclude connecting the OSBo board unless you figure out how to use cables to make the connection.
If you figure out a way to have both the xbee and OSBo board connectd it should be pretty easy to write a plugin for the ospi software to handle controlling the irrigation.
Dan