I cannot promise anything in this field. It seems like its a major design decision and I dont think it will be implemented.
What you are forgetting is what happens after you swapped the pistol. Lets say we want to sprint and hit the dirt and crawl fast 25 meters. To keep it realistic, the weapon should be at least pushed to the rear in order not to hurt yourself and not to damage the weapon (dirt, fall impact etc.). It would be rather uneasy for common players to acknowledge addditional procedure before initiating fast reaction motion.
Also, if you would see a GM6 Lynx or a GPMG hanging on the sling you would also be probably irritated. A compromise is needed.
Regardless, to make up for it, as i already wrote in the forums one day, please note that the time difference between real-life rifle to pistol swap by a common soldier is only 1/8 s faster than it is happening now. This was speeded up to compensate for these simplifications that we do.
It's not that we dont know about things :)