I owned a 'Trailer in a Bag' recently, and it's a super light trailer. Sure, you can assemble it, but it can be a p.i.t.a. That said, it's super light, tracks amazingly well and if you're carrying a reasonably light bike I bet the Smart can handle it. I towed the bike with my Honda Civic Si (2005) and it seemed fine, but it has double the HP of the Smart.
There is an aftermarket tow package for the smart, but I imagine with the Smart ye just want the lightest load possible. Some Harleys can weigh a lot so it depends on your bike I guess.