Matt,
You're correct that if most of these small cells goes offline during a power outage, the remaining macro cells would not be able to handle the load well.
Data would be nearly useless and phone/texts may be sporadic.
I believe that when this happens, they should proactively block or limit video and file download/upload traffic as much as possible to make sure communications like calls and texts can go through with the highest success rate possible. Netflix and YouTube should never hinder more important communications in my opinion. Maybe it's as simple as putting a rate limit for each cellphone connected to these now overloaded sectors so no one can hog the cell capacity.
It would be pretty sweet though if small cells all had a linked power source following the same fiber paths that all hook back into a large battery backup or generator somewhere. Maybe 30-40 small cells can have backup power from one macro cell generator. I'm not sure if they're installed that way or not but it would ideal. Otherwise, you're losing 10 to 100x of the capacity of a cell network during power outages if the small cells go down.