There aren't a lot of "off-the-shelf" mounting systems for handlebar
bags that Rivendell could have chosen for their "Barsack". If you
start with a premise that Rivendell wanted to produce a handlebar bag
that would fit many bikes that they sell, without having to make bags
sized to the headtube (e.g. Gilles Berthoud makes bags that range from
170mm to 270mm in height) that require a decaleur, a handlebar mount
bag makes sense. The Nitto F-15 bag support is beautifully crafted
and it works well with this kind of handlebar bag.
I've owned a previous model Rivendell handlebar bag, the Baggins "Boxy
Bag", that used the Nitto F-15 rack. I think the system works well
and can be adapted easily to many bikes without getting into the
complexities of a decaleur. (By the way, I now have a Berthoud GB2886
handlebar bag with a Velo Orange classic decaleur).
Jim Cloud
Tucson, AZ