Finally received my SONOFF SNZB-04 ZigBee Wireless Door Window Sensor ON/ OFF Sync Status on e-WeLink Smart Home Security Works with ZigBee Bridge and got to take it apart and see what's under the hood. Actually came very quick, under 2 weeks with cheapest shipping from AliExpress SONOFF Factory Store, not counting on that to happen again though.
My hope is that these will be a less expensive hacking option than the more feature rich($$) SmartThings contact sensors.
Turns out it's a pretty basic Zigbee sensor with a CC2530, reed switch, reset switch, LED and a battery.
@markus' drivers show the reported open/close states, battery level and he's included a presence monitoring feature.
Anyhow, on with the show:
Although the gull wing style reed switch isn't as easy to connect to as the old glass style ones, it appears quite feasible to re-purpose these as dry contact sensors. Yahoo.