It is not currently possible to do this.
The current metadata storage uses OSGi preference API and doesn't offer
this level of customization of storage layout. As part of our server scalability
work I am currently designing and implementing a new metadata storage API.
This will enable metadata storage to be completely pluggable on the Orion
server, such as allowing database storage or arbitrary file system storage
formats. For default implementation of that new storage API I am looking
at a file system layout that is segmented by user, which looks close to
your picture below. For more details see this bug:
The guid-like generation then will be as a feature, which
can be switched on/off. This will give great flexibility for those who
wants to integrate with Orion at lowest level (file system) reliably. Also
the remapping at every request as well as the mapping management (bottleneck)
itself can be disabled, if it is not needed.
What do you think?