Why not train according to cadence and heart rate? HR is only a few seconds behind power in terms of monitoring what's going on.
As others have said, don't overthink it. I know it can be easy to get caught up in the numbers, but understand that Power is number derived from torque and cadence. The torque can changed based on crank length. Not to mention that weight and air resistance increase/decrease the power required to maintain a certain pace. It's simply not a precise measure of anything.
Your HR can probably tell you more about how much training you are doing than power numbers.
Think about it this way, if a set of twins were training. They have different levels of fitness. One rode CX all winter and the other one rode the couch. If they wanted to get the same amount of work out of a training session, which would ensure that they trained their bodies equally?
- Training to Power
- Training to HR
I would say that the latter would be a better way to make sure that they did the prescribed amount of work. Why? Let's say we tune the program to fit the out-of-shape twin, then the in-shape twin isn't getting enough work. Vice-versa and the out-of-shape twin is working too hard.
What's the purpose of this training? To hit certain power numbers or to train your engine? Your engine let's you know how it's doing via the HR. Power is just a way to show off how well your engine is performing. Train using HR. Evaluate progress using Power.