If you disable their "move" and "target" AI, and put them in combat mode, they will stay down. However, once the shooting starts, they tend to pop back up eventually.
unit disableAI "move"
unit disableAI "target"
unit setcombatmode "combat"
unit setunitpos "up"
unit switchmove "crouch"
Only problem is that they can never move again, of course. But this works great for units in a fixed position.