Thanks. I followed your advice and emailed the suggestion in.
When writing it up, I added the idea of rather than just mirroring Osc A or B one octave down, also giving the option of just selecting a Triangle, Pulse, or Saw wave one octave down, with ideally the ability to set the pulse width of the pulse. Since there are dedicated buttons for the wave types on Osc B, I figured a user could use those to select the sub wave type. From a sound design point of view, since the sub is often just a simple wave on other machines, having that here as well rather than a mirrored version of A or B might be advantageous.
I suppose you would also need some way to adjust the mix of the sub waves vs the other waves, which could perhaps with done with the noise knob and something in global row 3.
I'm so happy with my P10 regardless, but having a sub to work with would be a nice addition. Fingers crossed it is possible and something they would want to add.