Battery looks to just be a 3v... you could always connect a wall wart that supplies 3v in instead of the battery if you are worried about the battery life. Of course it looks like the leads are not long enough to be running outdoors for checking...
But interesting idea. Looking forward to seeing how it works. Outside of weather stations there seems to be few ZigBee/Z-Wave rain sensors, especially simple ones like what are used to detect rain for skylights or such.
The newer ST leak sensors have an additional pair of contacts on the top, with a bit of a dip designed to catch water. I suppose they could be used as wireless rain detectors, without any modification.
I really like your idea. Would be interested in mounting one in a vertically oriented piece of PVC pipe to keep the water ingress limited to vertical. My question would be how the battery and device logic would fair with those probes sittin in water for a day until it dried up or was manually tipped out of that depression. I understand some of these have an audible alarm that may need to be snipped.
Seems like a cheap solution to telling the irrigation controllers not to water.