I think under your circumstances your idea might work. I was thinking the ideal conditions that type of setup would be an extremely low current draw over long periods of time, or intermittent loads, where as you said, the electrical load is so small that the energy to run the engine overwhelms the energy to run the generator.

Here's the spec sheet for the Honda i was talking about

http://www.hondapowerequipment.com/eu1000.htm

Looking at their fuel consumption, it looks like running at 1/4 load is roughly 2x more efficient than running near full capacity. And if you assume a charging efficiency of 70%, and an inverter efficiency of around 85% (just rough estimates), somewhere there probably is a break even point where it would make sense to set it up that way. it's kinda late in the evening for math, but I'm pretty sure someone can figure out exactly where that point is.