This is awesome, I really like how github has been exposing so many API's lately so we can use their tools in whatever way we see fit by simply tying several api's together.
Hooking this into our CI server and using the Merge API (for keeping PR's up to date) is the next 20-minute task I will take on.
Note that separately CI-ing each individual pull request still consumes what in past years would be an enormous investment in build automation. Luckily this is increasingly becoming a SaaS thing.