For those other G1 Insight owners out there, staying in lean burn is based on throttle position...
Fantastic idea. $100 is more than I'd like to spend on this kind of mod but it's cheaper than aftermarket cruise control. I'll probably play with low-rate springs for a bit and see if I run into what you did.
EDIT: A 2-stage mechanism would be ideal for the Insight. The upper one would be best as a tactile-feedback (a click), with your resistance setup for the lower one.