If that is the case.
Just drill a sight hole in the back of the magazine right under where the mag well ends and you can just see without having to take the rifle off your shoulder.
Functionally, they are a solution looking for a problem.
-You shouldn't/won't be looking at them in combat.
-You should shoot until the mag runs dry then change.
-If you do a tactical reload for some reason, you should put the partial mag in a dump pouch, not with your full mags and if it gets to the point you need that mag, the number of rounds in it won't matter.
But if you are just talking about trying stuff for giggles, let us know how it turns out.