I've ridden Fern Pass to Neuschwanstein today.
I used the Garmin to pick up the power meter data, but I set up a BigRing configuration to use my Bkool Classic to calculate power. On the assumption that the power meter is accurate, at first I was sure the Bkool power was too low, but as the ride went on it seemed to sort itself out.
After the ride, I uploaded both files to my ride software and compared the power traces:
View attachment 348420
The power meter is the red trace, the Bkool is blue.
The summary at the top of the image shows that on average the Bkool was 2 watts lower, and the graph shows that most of the discrepancy came in the first 20 minutes. That corresponds quite closely with the time I took on the initial climb. The other point where it was reading low (around 41:00 to 43:00) was also on a climb.
My conclusion (based on a sample size of 1, and so utterly unreliable

) is that overall the Bkool's estimate of power when riding on BigRingVR will be good enough to be relied on, but on climbs taking more than a couple of minutes I will be working about 20 watts harder than the Bkool says.
I've also set up a configuration which assumes that I don't have a smart trainer, and uses the generic magnetic resistance profile to generate the power figures. I'll be testing that in due course.