BRL-CAD presently has no support for constraint and parametric objects. Parametric capability improves the capability of following a different (and most times more efficient) design methodology and hence would add value to the BRL-CAD modeling system.
The proposal is for the creation of a library (libpg ) which sits above librt doing equation resolution and storage of non-geometric object types and parametric data on demand. It would not affect the existing functioning, storage mechanism and raytracing thus keeping complete backward compatibility. Developer Document of exact architecture as well as extent of parametrization possible is available at
http://brlcad.org/wiki/Libpg_:_A_parametrics/constraint_library