Bonding jumpers can only be max 6' 250.102(E)
... but see what you make of 250.120(C)
(C) Equipment Grounding Conductors Smaller Than 6 AWG. Equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by a raceway or cable armor except where run in hollow spaces of walls or partitions, where not subject to physical damage, or where protected from physical damage.
... so you can run a ECG outside a raceway/cable if it is 6 or larger or otherwise protected from damage.
To be honest I have never seen it.
