Hi Hythim,
Cisco UDS is a simple repository, I am not aware of a way to allow custom access to certain fields of a given directory ( mobile cell phone number field in the user_id record)
You may point your _cisco-uds SRV to a firewall or load balancer, that can intercept the request and via security profiles or iRules to in order to perform the following:
1 - check if the HTTP_REQUEST comes from a Jabber CLient
2. if true, allow connection to CUCM server/UDS url
3. If false, drop connection
As for the .xml encryption, that will be a global Workstation encryption solution for all the files - I imagine the encryption requirement is there not only for Jabber files, so it should not be something UC specific form the service point of view.