dust, smoke, grease

Is this an issue with Magnepans too? I know they are not electrostats, but seem to always be mentioned in the same vein of planar speakers.

The reason I ask about these issues is that we would likely put them in an open living area in continuity with the kitchen. How often would you have to vacuum them to have them sounding new?

yes, the speakers become the equivalent of electrostatic air cleaners that are in many forced air heating and cooling systems. One way to mitigate against that is to use a humidifier, like the Vetta which also is an air washer. Using the latter will also control static.