There was a proposal last month to the FAA from Amazon to establish a "drone airspace" below 400 ft agl. plus an exclusion zone from 400 - 500ft agl to separate drone traffic from other air traffic (airports being excluded from this drone airspace for obvious reasons).
https://www.documentcloud.org/docume...-the-safe.html
Now, providing private airstrips were also excluded etc. I could see this being feasible for fix wing traffic which would be unlikely to be flying this low outside of an airfield - although it could make an off-airfield emergency landing even more "exciting" that it needs to be!
However, my immediate reaction was this wouldn't work well for rotary traffic which may have very legitimate reasons for being below 500ft agl. outside of an airfield.
Matthew