Mmmm.... We'll need a 20 W capable resistor (which usually comes as ceramic one and it is not tiny enough). Welll actually, not so big, but you know, the safety margins.....
And the heat would be dissipated not only in the atty, but on the resistor, INSIDE the iStick......even if it had plenty of room there, which it hasn't...
Sorry, we cannot do that, it is not a good idea.
Another thing is to change the way of controlling things by the chip, a slight change. If it is possible, and if it has a way to do that. On paper, it is as simple as to change the voltage used to calculate power from the one now used (average) to a new one calculated as the squared root of the product of the voltage measurement and the peak voltage. Because that's precisely the RMS voltage.....
But I lack of any expertise in re-programming chip modules (the so-called firmware) and after all it is a propietary design...who knows about its guts? Me not, ........
Any rocket engineer happens to be lost here? It is just a chip!.......