Sorry for the late answer.
This is exactly what I want to do.
I just want to display custom set of modules(~IUs for simplicity) for the clients based on their access rights. I also need an administration interface where the module developers can publish their modules(module metadata), and the administrators can manage the user's priviliges. The priviliges, users, and metadata for the modules will be stored in a database and I will dinamically generate custom content.xmls for the clients from the database. When the provisioning is initiated, the client downloads this tailored content.xml, and the invokes the IMetaDataRepositoryManager#loadRepository on the local copy. Currenty I am working on this content.xml generator. The problem is that I think it is not a standardized format for the metadata, but I hope that it won't change in the near future. (I didn't find any schema definition in the content.xml generated by Eclipse) It would be a great improvement in the future to standardize the format of a P2 repository. :)