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I agree with you Adam - if you keep the mirror above the dew point temperature then it isn't a problem. The dew shield works by keeping a pocket of warm air above the mirror or corrector plate however as the night progresses this pocket also cools and in my case dew forms on the primary.
Most heaters don't account for the relative humidity so are as you call them "dumb" however you'll know from your dew point calculations that if you keep the mirror slightly above ambient you won't get dew formation no matter what the relative humidity is and thermal currents are not going to be an issue unless you overdo the heating. This principle is successfully applied to refractors using heater straps on the dew shield however in a big Newt you'd have to apply a fair bit of heat to the tube to heat the air across the mirror face and it would be difficult to get the temperature uniform. If you set up hot and cold zones in the tube you are going to get thermal currents forming. You also want to ensure you apply the heating to the mirror in a way that won't cause it to deform.
Peter
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