right. i've just had a wee play with the knowsabout command. it seems loons will always fire first at the enemy they know the most about. threat assessment appears to go on too, as well as depending on which direction the loon happens to be facing, when it comes to the loon deciding what to shoot at.
(thinking while typing here...) it follows then that if you have an array of all the enemy units on the map, then go through that array checking the corresponding knowsabout level for each of your law loons, keeping the highest knowsabout level in a variable, you then know what the law loon is likely to be targetting.
stopping the law loon from firing at it is another matter entirely. removing the ammo is the only way i can think of which won't involve disabling the loon altogether. but then, how much ammo do you return to the loon when it's not a truck? you'd need to keep a record of each law loon's ammo count. that coupled with constantly checking arrays would slow the game down to a crawl i suspect.
the golden rule of flashpoint editing i have found, is when in doubt, cheat. setcaptive all of your enemy trucks from the beginning. they'll never be targetted, but then you skip over the law-wasting problem.