photographing cyclists...

I'll be photographing a peloton of MAMILs (Middle-Aged Men In Lycra) for a product launch next week, and to get the action shots on the road, I'll be hanging out of the back of a Toyota Hilux in a climbing harness.
I've not shot this way before, so I'd appreciate a heads-up if you think I'm going about it the wrong way...

I'm pretty much set on using a 580EXII, either on a tall bracket with TTL, or superclamped to the tailgate and radio triggered manual, with sto-fen or small softbox to light the rider(s).

Have I missed/overlooked anything that you feel might be important? All advice appreciated!
Cheers.

How many riders?

Sounds nuts. Closed road? (I hope)

Anywhere from 8 - 11 riders; they're all actual cyclists rather than models and so they've not been 'booked' but will be riding their usual Sunday ride, except I'll be asking them to ride up and down, over and over, til I have the shots I need.
Oh, and no, it's not a closed road as such, but a 'gated' road, which is a very quiet country road where you literally have to stop your vehicle and get out to open and shut livestock gates to drive on it, and so almost nobody ever does. The next best thing to - and £££ cheaper than - a road closure and much more realistic looking than shooting cyclists on a test track or motorsport circuit.