Many thanks for your suggestion and support for TrainingPeaks Virtual.

I'm delighted to say that this feature is already fully supported by TrainingPeaks Virtual. Simply select the "Join" option from the action bar at the bottom of the screen (or if using a keyboard, you can press the hotkey 'J' for (J)oin), and you can then join any pacer at any time. You can also use this to join any friends you follow, or any team mates if in a team.