I noticed a thread on the TrainingPeaks support forums that suggests they now support syncing of "Garmin Wellness" data to your TrainingPeaks account via a third-party service called FitnessSyncer.
Has anyone tried this out? Can any TrainingPeaks users verify if it's possible to set-up your various accounts so that the data from a Garmin Index Scale gets uploaded to Garmin Connect and then automatically synced over to TrainingPeaks? And if this works, could you confirm what data it syncs (e.g. weight vs all the extended body composition metrics).
Disclaimer: I have no affiliation with FitnessSyncer whatsoever, I'm just a normal user trying to get data to TrainingPeaks. I haven't used it and cannot vouch for it as a product/service.