Why the need of such work-around? Is this a legacy part issue?
Anyway, I think you can get this to work by creating an analysis feature in the assembly that measures the distance, setting up an associated parameter in the flexible component definition, and defining a relation for the flexible component to the effect of "associated parameter = assembly measure:length".
Examine this thread for discussion about associated parameters: Is there an automated way to toggle between constraint sets without using Family Tables?
But this is so tedious compared to the direct method of controlling a flexible dimension by the "distance" measure...
