Currently when you ride an event, the data in the start pen while on the virtual trainer is not recorded at all.
Lets say you start riding before the event A, say 10-15 minutes before, and then auto-join the event 5 minutes before and do a warmup B, and then you ride the event C. Zwift will record A as one ride, it will only record the time for B but no other data (and will only record the time implicitly in the timestamp of file C), and finally it will record C with full data but the first data point in that file will be the start of the event C (even if the timestamp of the file will be at the start of B) so the warmup in the start pen is lost. No data is recorded; no power, cadence, heart rate data is recorded. Strava will then ignore that the timestamp of C includes the time in B and just go off on the first actual datapoint. The time difference between Zwift and Strava is normally 5 minutes; and again, all other data is not even recorded by Zwift.
Since the avatar is on a virtual trainer then that avatar should also be on Zwift; somewhere on a short 500 m velodrome-style loop. If the rider is indeed warming up (and not just sitting in their bike waiting for the event to start) and there is power/cadence/HR data to record then the fake GPS data for the avatar of the avatar could be that of the velodrome.
When the event starts, Zwift would save that warmup B separately from the pre-ride A and the actual event C.