Yup, my helmet setup is virtually the same as the picture posted by 'linestretch' (nice pic), except the video camera on my setup is totally enclosed with the box hinging at the top to access the vid.
I think I would have to do something like the picture to attach an external flash unit. But just done a bit more digging and it looks like the EOS5 does not support high speed sync anyway, so pop-up or external - 1/200th looks like it is the fastest possible - if anyone else knows different please let me know.
Re-charge time for the pop-up is approx 1 shot every 2 seconds so would have limited use for tandem work but could be used quite well for RW.
Ah well, thought I was buying a stills camera to see me good for a while, but already found a limitation.
Loads of good info in the replys, thanks to all the advise and looking forward to reading any extra info that follows.