Excellent article. Thank you very much. Your opening of how you worked in many places and faced the same issues (reporting, etc.) can, I am sure, be related to by many other IT professionals.
Your concern with middle tier code crashing when adding your audit columns is completely valid, albeit, a customized concern. As you pointed out, every situation is different. For example, some middle tier code have direct attribute (table columns) to property mappings (class/object properties). There must have been a ton of modifications to your sprocs for your situation.
Your detailed model of how you went about this challenge is excellent and again, I thank you very much.