You could use the Insert Beat/Measure function to do this.
Record a couple bars leading up to the free time section so you can get the feel right.
Do a couple takes of the free time section until it sounds/feels how you want.
Figure out what time signature the free time section would probably be if you played it rigidly to a metronome and set the section up to that signature.
Now wherever there is a beat throughout the free time section right click the Sonar Timeline, select Insert Beat/Measure (or whatever it is called) and in the dialog that pops up select the appropriate setting.
Now when you get to that point in the tune to record your keeper takes the metronome will reflect the "free time" you've program, any MIDI stuff (including the PRV) will follow that tempo and the rest of the project will stay in the proper signatures and timing.
There may be other solutions but that is what I would do. Cheers.