Well, there is one good biological example of constraint and perfect conservation - the genetic code. That varies very little and presumably has huge constraints mechanisms built in to preserve it. Those features that we have selected to be parts of the Bauplan are highly conserved, so of course they are not changed. Constraint outwith the Bauplan . At a second level we have the other characteristics, those which we have said are not part of the Bauplan.
Constraint plays a lesser role here.
One drawback is that this division immediately splits characters into two leagues and necessitates first of all some sort of judgement, presumably applied externally by us, as to whether something is Bauplan or is not and secondly we need a mechanism to apply constraints. Below this things are fairly easy to change, above this things are more difficult. The more fundamental a thing is, or the more necessary, then the bigger the lock on its chastity belt. Epigenetic organisation of embryonic development. If you are a preformationist form already exists in the egg.