I was told about a really good idea for a hand throttle while at work today.
Take a thumb controlled bicycle shifter and hook it up to your throttle linkage.
The "clicks" that would hold the gears on a bike will hold your throttle at an even RPM.
Just mount the shift control under the shifter knob and run the cable.
Edit: I think a twist style bike shifter should work also, but the person who told me hasn't ever done it that way.