My point here is that the moment you get MCe, you (as in the player) should already remember that the last two tiles of your LOS are courtesy of MCe. I don't see a point in having those highlighted at all. It's not as though the physical representation of the LOS isn't increased when you took a point of MCe.
I am very hesitant with mucking around with the screen-map to generate specific effects -- all effects that are done in graphics mode need to be backported to ASCII mode, and Kornel has the view of not implementing background highlights in ASCII mode, ever.
I think that the current amount of information provided is sufficient, unlike say the number of shots in a rapid fire weapon as displayed when you have TH. That is not reflected anywhere, so suddenly your Pancor Jackhammer under 2xTH fires a volley of 5 instead of the highlighted 3, or when your chaingun suddenly fires 6 shots instead of 4.