As you can read elsewhere, this is a long overdue isse - and nothing will probably happen anytime soon. At least I've lost faith in Garmin Connect and have moved to runningfreeonline.
In their health section you can input weight, body fat and resting heart rate. You can even delete wrong inputs :D
Guess I'll check back here once in a while - just to see if they ever wake up or let it all go down. What a waste, though....
Only way I have found to enter all values manually (cause I've another Tanita scale) is to build & upload FIT file myself 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.