If you use a fleet of drones at once, presumably you could get an even larger synthetic aperture (radar from each drone bounches off the ground and is picked up by all other drones)? Contrary to movement-based synthetic aperture this would be movement+many-receivers?
SAR from a drone would be peanuts compared to the aperture available from a space plane. On the other hand, if one is studying a specific area, illuminating with a drone and passive SAR with a space plane might be very effective?