I have a WeMo wall switch that says I should not install if I do not have a neutral wire. What would be the result if I connected to ground?
There are two main problems:
Since building codes doesn't allow such usage, any inspector who sees it is going to flag it and likely want to inspect a lot of other wiring.
If used in a circuit which is GFCI/RCD protected, any significant current the device would feed through its ground-connected neutral would likely cause the protection device to trip.
I don't think there's any technical reason why it shouldn't be possible for a manufacturer to create a class of switches which would be approved for use in such fashion [ensuring that such switches would never pass more than a fraction of a milliamp through the neutral lead even at startup would require extra circuitry, but that would not be an insurmountable problem.] I would expect, however, that manufacturers have no motive to build devices which regulators would not recognize, and regulators have no motive to recognize devices that don't exist.