Measuring commanded servo position, is this a no-no?
Sounds like it works.. you're measuring an average voltage. The only thing I'd be concerned about is does it drop at any given setting as the rx battery voltage drops? Since you're technically measuring time and voltage instead of just time, the voltage is going to be another variable. You'll also get different readings from meters with different sampling rates and A/D methods.
As flatfour said, there's no chance of hurting the rx unless you fatprobe something.