Glamdring, I was under the impression safety equipment was being worn (even a Shinai can seriously hurt) so I figured bokken was a better choice... more similar handling characteristics, plus you could paint the edge for markings in the way simunitions will leave a print. If no "armor" then I'd still opt for a weighted foam bokken over a shinai for safety and the edge painting... try painting a shinai and you'll get a big smear on your target!
Hell, I'd propose a safe version of this as an alterative to paintball! Simunitions ought to be an airsofter's dream while swordsmen get to play out their fantasy of taking out a gunmen with only his blade.