Long Polling
It seems like there is an api method (private?) that allows for long polling on home.nest.com. This still appears to work since I migrated my Nest account. homebridge-nest also makes use of it:

https://github.com/chrisjshull/homebridge-nest/blob/744b48474f126060833e5ee97330112f652bc48c/lib/nest-connection.js#L415-L441

Likely will need to subscribe to different object ids than what homebridge-nest is requesting, but this should at the very least let you know when a motion/doorbell event comes in. 