I don't know much about your trainer, but it might be calibrated so's it knows that it takes X power to turn the roller at Y speed when Z resistance is dialed in. If there's no resistance adjustment, the #'s are even easier.
This gives me a flash of inspiration: Borrow a PT-equipped bike (wheel would be better) and ride your rollers or trainer on it. You should be able to find out power levels required to run it at different speeds - calibration, if you will. Fatal flaw: resistance may vary with temperature (or humidity, or phase of the moon, etc...)