I'm not exactly sure what you are asking or trying to acomplish...
In any simple "leveraged" type situation there will be a small ammount of suspension induced rate change. Placing a bag into a situation where the rate change is drastic is not a concern because of it's limited mounting options. You would gain more advantage concerning yourself with a slightly rising-rate shock linkage rather than a linear bag mount. The interesting thing about a linear rate bag mount is that the bag is a progressive rate part to begin with and calculating out the ammount of rate is best left to the Firestone tech dept. You can lessen the effect of the rising rate by mounting a small "expansion" tank inline with the bag supply line, that way when the bag compresses the air has somewhere to go to lessen the rate change, but that gets interesting too.