Hi there are indeed P90 mag prouches which you can attach to modular gear. This prouches are made by a firm with the name VIPER you can find them here.
http://www.begadishop.de/catalog/pro...oducts_id/2050
I am sorry it is in German.
or you take a look here
http://www.tigertactical.com/gallery/
When you are searching for a good sling I would take this one.
http://www.stargate6.com/sling.htm
About the BHI pop-flare pouches I wouldn`t recomend them. It is possible to fit a mag but it is not the best solution.