Reloading from Esstac’s KYWI magazine pouches is very fast. Reloading from a horizontal magazine pouch can be fast. So… reloading from a horizontal KYWI will likely break (or at least tamper with) the laws of physics.
Esstac recently shared an image of a micro chest rig they are testing that features 2 horizontal 5.56 KYWIs paired with 2 vertical pistol KYWIs. The rig may see production if there is enough interest.