Theoretically Speaking (triangulated four link)
 

what would you expect from a setup that consisted of tubular style, stock positioned trailing arms, combined with a parallel bar on the outside of the frame for the bags to mount.

I look at pics of triangulated four links that have the two outboard bars (one on each side), paired with the shorter triangle placed bars going to a crossmember in the middle.

aren't a pair of trailing arms doing essentially the same thing as those "triangle bars" in a four link. minus the two outboard bars? i.e.. fixed axle-->fixed frame

im assuming length has something to do with it, but can't seem to picture what that would do through the suspension travel.

would this work if the parallel bars and trailing arms were a certain ratio length wise to each other?

Re: Theoretically Speaking (triangulated four link)
 

Four bars only work if all the ends are able to pivot. With stock trailing arms that wont happen so the system would bind as soon as it tried to move.
Jimmy