Hey Paul,
For a drop leg setup you may want to consider the Costa Leg Rig. It will hold rifle and pistol mags.
Costa Leg Rig
Another setup that I have thought about trying out is the Raven Concealment ModuLoader. This one runs off the belt.
ModuLoader Frames
Both of these use HGSI Tacos and I think that of the two, only the Moduloader is user configurable.
One thing I would keep in mind for using a drop leg setup is that if you are trying to reload on the move it may be a little more difficult since the mags are moving with your leg, and your hand will be searching around trying to get to the next mag.
I would also take a look at what the top guys in the matches are using and maybe run what they run. Or get some feedback from them.