I try to always have a designated reload pouch on all my mag carrying gear. This is the pouch that provides the fastest reload. It is reserved only for emergency reloads until all other pouches are exhausted. Typically, this pouch is on my belt, but there are times when wearing the belt isn’t practical.
The key to the designated pouch is that it offers a faster, simpler reload than the other pouches. If you are using magazine pouches that have an open top or you can attach an open top pouch to your rig, then designating an emergency reload pouch is as simple as deciding which pouch it will be. If you are using flap covered magazine pouches, you can still designate one as an emergency reload pouch by simply leaving the flap open.
So, I have been running the UW Gear Minuteman MKII with the left most pouch’s (typically the fastest reload for a right handed shooter) flap tucked behind the magazine. The Minuteman MKII pouches are tall enough and fit to the specific magazine that there is basically no way it is going to come out before you want it to. Also, the flap tucked behind the mag provides some extra tension to keep it in place.
This means I have reserve mags in covered pouches that can’t be lost but that I have one magazine that is more quickly accessible. So, even if that mag is lost somehow (I doubt it could ever happen unless you were turned upside down), it would be bad but not catastrophic because the other 2 or 3 will still be retained. I am willing to take that risk to have 1 slightly faster reload.
You have to be sure that your magazine pouches provide enough retention with the flap tucked away before you consider using this technique. It will work best with single magazine pouches but there are doubles out there that will work. Next time you are at the range, give it a try.