If you do go the fencing route, be sure to give the player the option to blow his way through - and if he does, the AI to react in the appropriate manner!
I remember one of the final missions in OFP:CWC where you as Gastovski were to infiltrate General Guba's manor on Kolgujev and steal some information on the nuclear missiles (man, that whole campaign kicked ass...) -> you started behind a truck going in through the gates, and you had to decide how to proceed from there. Run in, guns blazing? Stealthily take out the sentries along the whole fence and then sneak in? Blow a hole in the wall and hope you can get to cover before that BMP arrives? Try to go around the fence altogether? (no swimming at this point! But I think you could still walk into the water and get around the fence...).
Anyway, give the player some options, and make sure to outfit him properly (no AT weapons though, unless you want to turn it into a fragfest >_<) : sniper rifles, silenced guns, satchel charges, mines...
Oh, and be careful with using a simple Detected by trigger: due to a bug, the trigger will activate even if you cap someone in the head with a silenced M9 50 meters from the closest, ungrouped enemy -> instead use something like {_x knowsabout Player > 3} count thisList > 0 as a trigger (activated by OPFOR), which will activate when more than zero people know more than "3" about the player (they've probably seen him/heard him at that point).
Good luck!
Wolfrug out.