Nice idea, but would it work?
Answer? Well, they had an opportunity in the 90's and the 'spotters' failed.
Those peace loving people from an Island not far from the West Coast dug a big hole and filled it will mortar tubes, right next to where the spotters congregate. The result, spotters didn't notice.
End result a series of load bangs and 3 flew over the perrimeter fence. Luckily, nothing went bang when it landed.
The UK had been a target for Irish terrorism for years. The mortar episode wasn't the first go at LHR, and in none of the previous attacks (Car bombs in car parks as I recall) did spotters help in any way shape or form.
In fact after the mortar attacks, they still invaded LHR daily, meaning we had to divert resources to deal with them as well as everything else.
In short, it's a nice theory, but reality doesn't support it. Given the amount of extra police time and effort put in to deal with them verus the lack of result, is it worth the effort in administering it? No.