I don't know of functions to do an initiative check via scripting. Maybe NWNX has functions for it. I guess you could check each player's DEX modifier, if they have improved initiative, superior initiative, blooded, or thug feats and add 1d20 vs. a DC set on the trigger.
I thought about having spells like premonition and clairvoyance/clairaudience grant initiative bonuses once. That's when I realized it's hardcoded, and that it wouldn't quite accomplish what I really wanted it for: when you are attacked by an enemy from stealth, they always win initiative, because you couldn't see them.
This allows HiPS spammers to attack and run away before you can do anything back to them. If they also have epic dodge, it ensures you can never touch them. I used to think they could attack AND run away because HiPS builds typically had high DEX and therefore were usually winning the initiative roll, but found out later they always win if you can't see them before they attack. Combined with the ability to spam HiPS, but for players without keen sense to be able to get multiple detect rolls per round, it is a very frustrating mechanic in NWN for me.
Anyway, I like your idea of having a trigger that does an initiative check. Those that fail the check would be immobilized for a round while the statues come alive, or an enemy drops down from the rafters. It's like a trap, but involving enemies instead of poison or some form of damage or effect.