It is possible to enter all values manually by building & uploading a FIT file using the Java tool provided with the FIT SDK.
Working great :D
It's a bit boring to do, maybe I will make a tool do this automatically when I'll have more time.
I can explain how to do this if sombody is interested.
Can you use this to overwrite an existing set of measurements by changing the the date and/or use to edit existing measurements too? Would you please try and find time (you can skip Thanksgiving meal and work on it :D) to write this. It's obvios Garmin isn't going to do anything any time soon. I would be willing to pledge PayPal $5 or so for this.