Ahh, re-read your first post. You could try assigning some button from your mpc to it. Check out controllers/surfaces under options and select the generic control surface (or something like that). Gon to Tools -> generic control surface, try using the learn button.
Why do you need the record button to be triggered from the mpc? I believe it should work simply by first arming the track you want to record ("R" button) and then arming the big record button. When you click play, Sonar should start recording.
I personally prefer having my 2000XL as slave, makes everything less complicated and I think the workflow is better.