The disadvantage to this setup, that I see, is that the finger-lever connection is very poor. This disadvantage is shared by the seatpost and rail bracket configurations. If you add a Delta extender, most of it has to be removed for leg clearance and you get only one finger on the lever.
The only commercially available setups that actually form a good finger-lever connection are
A) the bar end, which can yaw right or left depending on the rider’s handedness, in conjunction with a delta extender,
B) the modified brake lever, which has been posted about recently, which zigzags to present a 2-3 finger surface to the rider,
and C) the slightly heavier, but more capable modified GBDS setup that George and I co-developed (photo here) for non-hopping, cross-country, and road riding applications. Ben P-S recently used that setup racing in Canada. This presents the lever to the hand just like on a normal bicycle handlebar. The GBDS handle is available through me, made by George.
Also I think Scot Cooper and Rick Hunter have developed something adjustable, for Coker long-distance riding, too extensive for smaller unis.
There is also someone recently on the forum who has done a Reeder-like handle-brake combination which looks really nice, but is a one-off so far.
There have been some paddles, or cross-bars, developed for the straight-on approach, Scot Bridgman did a one-off I think, perhaps Darren Bedord has one for sale.