Depending on the setup it doesn't even have to be a visual cue. With a simrig or wired DNI, you could have an audible cue (anything from simple chime to a short "What's with all these MFing empty mags in my MFing magwell"), a tactile cue (back of the support hand hot for low ammo and burning for empty), or even smell/taste for the weird people out there (when my gun's empty I smell lilacs and taste pepper). Any number of which should give a very very non-subtle clue that you're running out of ammo like a jackass.
Then again, I wish we had more official rules on the smartlink and settings for it. I think their should be optional settings for things like auto-ejecting spent clips, auto-switching ammunition (useful for blowing out glass with an Ex-Ex followed by an APDS called shot to target 8 holding that hostage), or even auto-changing fire modes in certain situations. Setting up parameters for the computer processes shouldn't be too hard, but then again that would go against the streamlining of the game.