The thing is that FirstBeat software runs on devices, not in Garmin Connect. If your activity does not "pass" through your device, then firstbeat's software has no means of calculating anything.
I wonder what would happen if I tried logging using both the 935 and ErgData - would it create a duplicate entry in my Connect account, merge the data, or discard one of the activities as redundant? I should give this a test run next time I'm rowing.
What does affect FirstBeat training metrics?
I did a pool swim class without HRM-Swim. Training effect: zero!
700m
Time: 45:10
Swim Time:23:47
Ok, HR is what measures the effort, but it would be good if there was some other metric perhaps based on recent similar workouts.