Refer to the diagram below. For a NuVinci n360 you have two shifting cables, the underdrive and overdrive. It seems to be it is possible to use a spring or counterweight to pull the underdrive and then use a conventional shifter for the overdrive (may require modification/consideration for full length of pull). Is there a fundamental reason this couldn't work? Have people tried? A few Googles turned up fruitless.
The difficulty I can see is the 120mm draw length of the cable. The system may need to be a bit more elegant than a single spring.
Open to suggestions (I hope this has been done before, success or otherwise)