Works consistent for me, but I have found you need to make their ROE 'weapons free' - even with 'return fire' they won't execute a fire at point.
So your best bet is to make the friendlies invisible (so they won't be automatically engaged) and have them set to hold fire or return fire (so they won't open up). You can always make them visible again with a trigger later if you want them to get mowed down if the player takes too long to respond.
Attached is your test mission with the 'fire at' point moved closer to the insurgent position (so they can actually fire at it), and a repeating trigger to have them fire in relatively short bursts. It never disables (so they'll keep firing at that point in bursts until they run out of ammo), but you could stop it at any time by disabling flags 50, 51 and 55. The timing kind of sucks but you can play around with it.
I think for it to work well in this particular situation you'd need a few different 'fire at' points to better follow the progress of the friendlies. It might also be nice to split the insurgents into multiple groups and have them firing at different positions so the aim point isn't so obvious.
Also the triggers I've put it indicate when they've activated - you'll see it takes a few seconds from the AI 'fire at point' task being activated until they actually start firing at that point. They cease fire as soon as soon as the stop flag is set, though. Just something to factor in when you're playing with the timings.
I've randomised the start time to be from 5-25 seconds from when the start flag (#50) is set - to make the bursts a little less predictable. testbed.miz