I have a feeling the AI can't target gamelogic, try invisible markers instead, or an invisible helicopter landing H.
Give it a go and see whether it works, you never know. Your basic idea of triggers, doTarget and doFire does work in principle.
Alternatively, if you can place the mortars accurately enough (I don't know the addon so this may not work), a simple doFire command may do the trick - the shells will land in the right place. In general the AI doesn't need a target to fire a single shot, and obviously you cann just repeat the process.