Puzzling question on Predefined Properties

We all know the Global Metadata service in KMC. It allows us to define additional metadata properties for a configurable namespace within KMC (can be activated for specific repositories, folders, documents). We can use the flexible user interface to display the properties and to maintain their values.

It appears however that not all repository managers (connectors) support the use of it. If you look at the standard Repository Managers in KMC, you will see that that the File System Repository Manager does NOT support the use of predefined properties.

Common sense tells me that this could be explained by the fact that the file system repository manager does not have a store to save the extra metadata. It is implemented a file system client, to read and write documents and to retrieve basic resource properties.

The same however holds true for the WEBDAV repository manager. Yet this one does seem to support the use of predefined properties.

So the question is :

What makes the WEBDAV repository manager support predefined properties and the file system repository manger not, cause both do not manage a local store for maintaining that information. Does the WEBDAV repository manager use an internal KMC repository to maintain that metadata ?

In addition :

In a scenario where you are building your own repository manager for a specific data source, what are the special requirements in its design and implementation in order for it to support predefined property values ? What if your remote data source is read-only, can you still build a repository manager for it that does support the maintenance of predefined properties ?