I believe the 30 rd mag is see thru. The 100 mag, I don't know. Might have to make some rounds near the end or like every 30 rounds, tracers, if you want to keep track real well.
I would think something that would be visible or perceptible to a shooter with a shouldered weapon would be desirable if it could be made reliable. For example, it should be fairly straightforward to include a counter which would be zeroed when the magazine was removed and would increment every time the action closed with a magazine installed. Even if the counter were electronic and had batteries that could go dead, a battery failure would not interfere with use of the weapon--just make it less convenient to determine how much ammo remained.