OK, I found a site that list a GVWR for the smart of 2,156 lb.
If this is correct,
2,156 lb - 1,600 lb = 556 lbs.
If you add a couple of 200# passengers, you are down to:
556 lbs - 400 lbs = 156 lbs.
Not very much!
I'm not the final authority on towing with a smart. These are just rough numbers. I'd like someone else to verify my calculation, but this should give you a VERY rough idea of what you can do.
Towing puts extra stress on the tires, wheels, brakes, transmission, frame, motor, etc.
Tow safe!