Running resistors above power rating, but with temperature within spec
I'd suggest that small SMD resistors would be questionable, but you certainly could use very small power resistors to achieve your goal. 

This type of power resistor might suit your need: http://www.vishay.com/docs/51055/d2to20.pdf

You have not been specific about your needs in terms of volume, but resistors like this have an SOA out to 140 degC. It would seem possible to drive this sort of device with a PWM to provide the temperature increase needed. 

I've also used two TO-220 power transistors clamped around a crystal to make a crystal oven at 80 degC...worked well. 

Depending on your application (and since you are already using a FET to drive the resistors) you could simply use the FET as the heat source. By holding VDD constant for an N-Channel FET at some voltage level you can dissipate all the heat required directly in the FET. All the same rules apply in terms of temperature, but most power FETs have an SOA out to at least 125 degC so 100 degC sounds pratical. You could sense the current through the FET using a small value resistor.

You could even use a linear power regulator in anything from TO-92 to TO-220 or even SMD D2PAK to provide the heat. Just configure in a constant current circuit and modulate it from the feedback pin divider.