Make them blackops and give then whatever gear you want ... different uniforms have different camo values, so they will be less likely to be spotted. If you put them on stealth, don't fire (until you want them to) and place them in a bush they should be fine. Use setcaptive as well of course.
Alternatively place them on an outlying island and teleport them in at the crucial moment. Place a gamelogic at the place you actually want them to be and use
loon1 setPos getPos gl_1
syntax not guaranteed.