Well, technically they should have the model you carry around before firing show the LAW collapsed, since opening it arms it and then all you have to do is puch a big button on the top I'm not sure I'd run around with that thing armed and strapped on my back
As far as unreloadable... I know some people have tried to simulate that with really long reload times, but that doesn't work perfectly.
Have you tried any of the new 1.85 features? Wasn't there supposed to be a way to activate script commands from inside the cpp now? If so, that would be your answer, simple don't have any LAW ammo anywhere and have two different weapons one for used LAWs and one for unused LAWs, put in an action for the player to arm the (unused) LAW.
For example...
you have a LAW weapon with no ammo and an action on it to "Arm LAW" which would change it to the usedLAW weapon and add 1 round of ammo. The usedLAW would have no "Arm Law" action on it so there would be no way to get more ammo without picking up another unused LAW.
I haven't really played around with that idea myself so I have no clue if it's even possible, but it's an idea and maybe that will get you started in the right direction.