I have prepared a script [1] which creates a stream network (using r.stream.order) and then calculates contributing flow area to each stream segment, both at the start and end points of each segment. There is a parameter to choose the area units desired.
If this is of interest to anyone, please download and try it. If there is positive feedback, I’d like to offer the script as an addon. (Or possibly, merge the capability into one of the r.stream.* modules)
Cheers,
Micha