Importantly, the Overwatch rule is designed to allow friendly fire, and to limit out of turn activity. This is a fine rule to model that.
Reserved Actions, however, are designed to do something else. In Flashing Steel and Raygun Gothic, the action being modelled is not modern soldiers hunched over machineguns with their fingers on the triggers. Instead it is guards standing in towers looking all around. It is the guards in the hallway, with nothing else to do, waiting for the heroes to appear – what else do guards do? Ceratinly not look at some distant spot to the exclusion of everywhere else.
Reserved Actions are just that: an expensive way to not act within your turn, but instead take one Action when it is needed. And that is the important point that came out of the forum conversation. You cannot use Reserved Actions to act within your own turn.
Here’s an example of what you cannot do with Reserved Actions: on your turn spend 2 Actions to Reserve (model gets token). Then move on and activate other figures. As your other figures move they provoke a Reserved Action from your opponent (in other words: overwatch). You cannot now return to the figure that has already finished activation and invoke the Reserve. His activation is over.