I've always used replays to check that my overlays are working fine, but I just realized that ATVO multiclass racing doesn't differentiate between classes ...
Replays are only partially supported, many things may not work. I was not aware that multi-class detection doesn't work (it sounds strange) but it may well be.
To check overlays I can recommend just joining a live session, or using our Telemetry Recorder to record a live session data and play it back later (without needing to have iRacing running).