I would assume the duct work gets bonded via the EGC to the dust collector motor and enclosure. This is similar to a duct heater. Static is a whole 'nother thing, particularly if this is a plastic duct. In some of the woodworking BBs they have discussions about this. The fear is "lightning" in the duct could cause an explosion. I would think simply bonding a metal duct would bleed off the static charge.
I do not know of any article that deals with this unless you can call it "pipe" 250.104(B)
I had computer issues the past month so I wasn't on line.
I have seen the plastic dust collectors have a bonding wire run along side of them to discharge the static when sucking up the dust, but I haven't seen too many metal duct collector systems. My guess is that the metal will bleed off any static as it starts to build up as long as the metal duct is attached to a grounded piece of equipment.
From what I can find on the subject, the general consensus seems to be that with that type of installation the bare wire should be run inside the plastic duct to actually be effective. Placing it outside supposedly does nothing to dissipate any static build up inside the duct, which apparently is where the dust ignition problems occur.