First off, a link to a nice lil' tutorial. In the beginning it does a pretty good job of explaining RTT: http://rbwhitaker.wikidot.com/render-to-texture
Really, this is a weighty question as it entirely depends on implementation and circumstance. For example, what's being rendered at one time? We know ArmA has a draw distance (or has had in the past), so it's safe to assume we're not rendering outside of that. Chances are, though, there's drawing being done all the way to that mark. That's a lot, but systems can handle it nowadays (especially with more advanced techniques like LOD- level of detail).
Another question is, what sort of 'angle' are we drawing at? Is it a full sphere around your character for that entire distance? Usually games will draw what's nearby behind you in case you turn quickly- you don't want your computer to have to quickly load a gun in your face behind you! The distance may be minimal there, though, as well to the sides. This would create a sort of 3-dimensional teardrop of rendering space. Now I'm not TOTALLY familiar with this stuff and I'm definitely getting out of my territory now, so anyone with more experience please correct me.
With all these to consider, the final point would then be this: if you're rendering an entirely NEW scene from across the island, then it'll be far more costly than simply rendering the same scene that's already rendered with some postprocessing effects. Therefore, all this:
Could be feasible. Remember though that these are all technically new scenes being rendered, and I'm not sure if you can actually share objects between rendered scenes.
If you wanted to have a command post that had camera effects from all across the island, though, that would be a much greater strain on the system.