So finally the MOXY data field from Connect IQ is allowed by Garmin to write the muscle oxygen data into the FIT file.
Here is proof positive. I turned on a BSX Insight and used a Garmin Edge 820. The BSX paired as a speed/cadence sensor but the MOXY data field showed the SmO2 and THb data perfectly. Uploading to Garmin Connect and VOILA it appeared.
Doesn’t import into SportTracks though. Suggesting there are yet more changes to the FIT file spec which only Garmin support. Expect problems with 3rd party software. Fit File Repair Tool will fix but no data imported to ST3.1