Hi, Author,
It is very simple to setup the parameter for derived class of AIS_Dimensions, e.g. AIS_LengthDimension, the new a AIS_LengthDimension, then using AIS_InteractiveContext::Display it.
But the customed ability of AIS_Dimensions provided by OCC is far from the requirement of a profesional dimension scheme.
Just as Stephane pinpoints, One must derive from it and implement one's own Compute/ComputeSelection virtual method.
Commenter-1