Sorry, yes, that was what I meant. Use the MIG tip to just heat the bolt. Not to weld to it.The MIG contact tip would get hot for sure (the contact tip is absolutely electrically hot) but that's a lot of cross sectional area and the tips are copper, not steel. So it won't weld together in any kind of way that would be able to provide much - if any - force transmission. That is a stick welder only method.
The MIG might make a good bolt heater though - it would avoid the problems involved when using a torch (burning or melting anything even remotely close to the area).
I've used the welder conventionally to weld to the broken bolt. But using just the tip, no wire, could be a way to provide heat-cycling, for instance, to try and loosen the rust bond.