Old take-off cables, chain links and/or a spoke or two if you have them. An old take-off tire or two. Whatever emergency spares you usually carry on your long unsupported rides - those are the kind of things that riders on supported rides might decide to leave at home, but that can get someone back on the road in just a few minutes.

Know the route ahead of time so you have an idea of how many times you'll be able to circuit your segment. You'll want to drive slowly enough that riders can hail you if they need you, but fast enough that a broke-down rider won't have to wait longer than necessary, and it'll be useful to be able to let people know about how long it will take for you to come back around on your next circuit.

And thanks for volunteering!