Shameless plug, but I have been writing such a framework for quite sometime now. It has all the self-defining goodness you ask for - you can edit the models (a model is a model) via the admin interface. Also has an integrated front-end and back-end, so you can seamlessly share models in the client and push updates etc.
Its complex and not documented. It is primarily built for an small business accounting + inventory app we publish (ERPNext, demo - http://demo.erpnext.com/)
https://github.com/webnotes/wnframework
Its complex and not documented. It is primarily built for an small business accounting + inventory app we publish (ERPNext, demo - http://demo.erpnext.com/)