You can make a unit freeze in the crouching position by using the following code on him:
loon1 setUnitPos "UP"
loon1 setBehaviour "Combat"
loon1 switchMove "Crouch"
loon1 disableAI "Move"
He will not stand up if he is firing, etc, but be warned that he cannot move from that position. Otherwise, execute everything but the disableAI command on him. Unfortunatly, he will generally stand up when he sees the enemy, but he can move. You could also try a doStop command, which might prevent him from standing.