Tim: Thanks for more specific information on what years are towable.
In days of old, all manual transmissions were lubricated internally by the turning gears "splashing" oil all over the inside to keep things lubed. No problems towing. Some auto trans are similarly lubed, or lubed by a pump on the output shaft. When the car is towed, the drive shaft turns the output shaft and any attached gears, so things get lubed.
Other transmissions have the fluid pump on the input shaft, so towing provides very limited lube. An aux pump can provide lube while towing, making them 4 down towable with the pump.
Other transmission designs are simply not easily modified. This includes shift on the fly 4WD because the transfer was has no neutral.
To the OP: The ONLY part of any remaining warranty that would be voided by 4 down towing is damage CAUSED by 4 down towing. Also, rear wheel dolly towing is NEVER advised because the car was not designed to run backwards faster than a few mph. Tire alignment is set for going forward.
With that said, buying a flat trailer is also cheaper than setting up the Miata for flat towing. Just make sure the trailer has brakes of some type - either surge brakes or drums with wiring for control from the tow vehicle. Actual towing should be simple.