I have a workaround installed now. It's not pretty though.
Created a second temperature sensor in my sketch and did the mathmetics there also. In Vera I monitor this device for +2 or -2.

     send(msg.setSensor(0).set(temperature,1));
      // Custom
      temp_diff=temperature-lastTemperature[0];
      Serial.println("Temperatures compared: ");
      Serial.println(temp_diff);
      send(msg.setSensor(1).set(temp_diff,1));

Setup: Arduino Dallas temperature sensor, in combination with Vera Serial Gateway.
I'm trying to compare LastTemperature value to CurrentTemperature so I can start the bathroomfan, when temperature rises. Doing so using Vera's 'temperature goes above/below' is too slow. The hot water pipe cools down too slowly.

Problem is, there is only 1 variable sent (CurrentTemperature). I can add custom variables to a temperature device in Vera, but can they be updated by my sketch / Vera Arduino nodes? If so how?