I could offer a section on the dashboard, where model D users could manually drop their CSV files (perhaps in a zip file, all together) and the server would identify the missing data and add it to the DB. Is this something any of you would like to use?

Absolutely! I spent a little over 24 hours away from home, so I have over 9100 samples to sync and that will likely take 15+ hours – hopefully it will be done by the time I need to leave for work tomorrow.
I’m not sure how the offline sync works now but couldn’t the model D itself upload a zip file (or even a simple tar file) of CSVs for offline data? It seems like it should be able to upload the 1MB of data for the 9100 samples much faster that way.

With regards to GPS antennas, I’ll look into other options. Do you have the part number or specs for the current antennas or know what the gain is?