I picked up a STAGES over the weekend and have just been for a quick Zwift workout. As T Y states above the only way to get ERG to work is to select the KICKR as the power source. From comparing the data from my Garmin, various apps and Zwift, the KICKR/Zwift combo was ~10-15w higher than the STAGES.
I must admit didn’t get round to testing having the STAGES control the KICKR, so I’ll give that a go next time.
It would be nice to be able to accurately compare my indoor suffering to my outdoor suffering!