Most vehicles would be unstable towing backwards because the wheels can turn and their is more weight in the front of the car so towing on a dolly results in to much weight at the back which could result in instability.
The OP got away with it because he locked the steering wheel although I would be concerned about the locked steering wheel getting loose in the future so that would always be an area of concern.
The miata has roughly a 50:50 weight distribution so for a miata unlike most vehicles the weight distribution is virtually the same regardless which end is at the front.  While it would be unstable for most vehicles it would not be in this case, assuming you can trust the steering wheel locking.  I would not want to count on the steering wheel locking or have to deal with a tow dolly personally.