Questions regarding the use of the Petzl Shunt as a professional rope access back-up device surfaced way back in 2001 following an extensive study by Lyon Equipment for the Health and Safety Executive in the UK.
The 2001 Lyon Report raised some serious concerns about the Shunt's dynamic performance (loosely translated the Shunts slippage on certain ropes under relatively low loads).
Furthermore the Petzl Shunt is not self-trailing during descent.
Descent 'trailing' was achieved by attaching an accessory cord through the small hole of the reverse of the cam - see fig 1 below listed under Backup Device. Note the Shunt's cord tether in the climbers left hand. The climber pulls on the cord to release the cam during descent.
Due to safety concerns, the use of the Shunt in this rope access configuration is largely superseded by more appropriate gear like the Petzl ASAP, ISC Rocker and DMM Buddy.
Enter tree climbers!
Like it or not we are the Johnny-come-lately's in this discussion!
The cord tether on the Shunt is not such a big deal for tree climbers since we do not employ a secondary line but the slippage under load on a single (or double) line may present a serious problem.
According to the tests conducted by IRATA, the longer the cows tail or lanyard the greater the slippage particularly on thinner diameter ropes.
Matt Glen has asked if a carabiner connection might do the trick, presumably connected to the Shunt as a back-up above a hand ascender (thereby eliminating the need for a Croll in the 'Frog' system).
Judging by the amount of slippage that occurs in the tests you would have to question whether the Shunt, in the above configuration, even with the shorter connection via the carabiner, offered any backup whatsoever.
Hypothetically, under dynamic force, the hand ascender might tear through the rope sheath.
In that scenario the Shunt backup would then come into play.
But judging by the amount of slippage apparent in the videos the Shunt would slide, rather quickly, into the torn section of rope.
How effective would the Shunt be as an alternative safety backup in the 'Frog' configuration? Not very effective, one would think!
Technical Advisory Committee? Perhaps they have the answer?