Well, fully open system with open api will be the easiest to get support. Nest is not one of those. Doorbird i.e. has fully open api, you actually can without an homey app for it make it trigger external listener. You literary copy get url to doorbird. Other way around, to get stream from it you just paste specially crafted url to Internet browser and voilà. Unfortunately Nest will never do this kind of support as it relies on the subscription.
Netatmo announced doorbell also which will most likely be fully open. Nonetheless, I just want to say when something is closed it will not pickup community like some open devices for sure.
Aside of this, I think Nest will get support on homey as soon it picks up popularity and some developer finds is interesting to do. You can make request on the official part of the forum for requests following form mentioned in the lower post.
