Constitution would be simply new entry in mission files
Policies would be simply listed here.
Any policy, that is marked as constitutional would be more expensive to change or cancel.
This way countries would feel more unique since some policies would be harder to adjust and cancel.
Cost of implementation would be simply scaled by sum of voter membership*happiness, if policy affects voter groups happiness directly.
Hated policy would be harder to implement, and loved policy would be easier to implement.
More political capital makes all changes easier, but this one would be laser focused on voter groups, that are directly affected by policy to be implemented.
If your country is 80% liberal, then it should be harder to implement policies, that anger liberals.