Idle control is done by current sense.  This is usually an inductive pickup current sensor feeding an electronic control that controls the gas throttle.  

Second, idle control is intended for usage like construction sites where electric hand saws, and such are used.  These are brush type motors and are okay with the voltage and frequency ramp up associated with generator idle control.

I would not recommend using generator idle control to run syncronous motors like refridgerators or window air conditioners.  They have high starting current and don't usually like the voltage/frequency ramp up associated with a generator idle control.

Seriously though..  A year ago I spent considerable time on the internet trying to find this solution to my own generator and the only thing I found was the repair schematics to a commercial version, it used a cpu and a very complicated circuit. I think by now I have misplaced that info as well.

It really does look like there ought to be a simple way of accomplishing that. When the generator is idling, it's going to need a voltage injected into the line to sense a load. When it senses a load, it's going to need a current transformer or sensor inductor to detect if the load is still present while it is driving the load. I suspect someone with Stamp or PIC skills would have a ball with this.

Maybe rig up a solenoid from a car carburetor?  The kind that increases the idle speed when the a/c is turned on?

To activate that, anything to measure current would do.  Maybe a foot of #18 wire as a shunt, with a small hobby/audio transformer across it, could be adjusted to produce a volt or two in the secondary, which could be rectified and used to turn on an op amp or transistor, that would then activate the coil and increase the speed of the unit.