You would have the most fuel-efficient tow vehicle on four wheels. Of course, your mileage would vary... regardless of the light trailer weight, aero would suffer, stopping distances would lengthen, acceleration would be more leisurely, etc. etc. etc. What kind of hit you would take mileage-wise, who knows.
Attaching the hitch might be problematic, although others here have done it. There's not much to anchor the hitch back there, so normal hitches may need to be modified to find a more substantial attachment. Be sure and do a search, IIRC, others have done it and have been happy with their results... but the hitch has been used mostly as, say, a bike rack attachment. There are lightweight motorcycle trailers that would probably be perfect for the Insight...
Do keep us posted, but as long as you're a reasonable driver and pay attention to the Insights limitations, I can't imagine why you couldn't do this...