The CA V3 measures human watts using crank rpm and a torque sensor giving totals for HWhrs, AvgHW, and AvgRPM. There's a little HW bargraph on the main screen. This stuff and the usual bike info are sent out the serial port where the data steam can be logged using an Analogger or a phone with a USB port. If you log the CA data with an Analogger, it will tag the data stream with GPS data. So - for a usual PAS bicycle setup w/torque sensor, the CA already logs motor and human power output. The
ebikes.ca Trip Analyzer plots all this CA data with maps and stuff.
Since the CA can be used with any controller, your challenge would be the torque measurement, but a BeamTS could be used with a custom mount. An issue would be that all the friction losses of the elliptical pedal thingies would not be reflected in the torque/rpm calculations, but the significance of that error vs the other data is another matter.
In any case, all CAs log the bike data so if you can get a time-based data log from your Garmin, you can play it against a CA log and examine the results ex post facto.
Not the level of integration you seek, but doable with a bit of effort...