This is what I would do in your situation.
Keep the content model, but only the model, not any of the other customizations. That means all your content will be intact, indexing will work, and you can upgrade according to recommended procedures. The result is for the end user that you want see any custom properties, by removing customizations like webclient-custom-config.xml you are essentially hiding for end user. Whats important is that data integrity is intact.
-Take a clean 2.2 alfresco.war, and replace any customized alfresco.war.
-Remove all customization stuff out of tomcat/share/classes/alfresco/extension
-In extension, keep the data model.
-Now restart 2.2, make sure everything works
-Start the upgrade procedure as outlined in the wiki article I linked to.
-> End result should be that you have an upgraded environment, default install, with some deprecated properties, constraints.
When would this method not work? I can think if instead of creating your own model, your external developers modified alfresco default model. That would definitely complicate/break things.
There may be other stuff in your model that makes my approach harder, but then I need to see it.