You want to configure ProxySG appliance rules to control traffic using computer Hostname.

1.  Make sure the internal DNS server is able to resolve a client IP address to computer Hostname.
2. To confirm, use nslookup "client.ip.address" or "ping -a client.ip.address" to resolve to client host name.
   Example: client host: CS764, Domain: kldev.bluecoat.com
   
   nslookup 10.107.17.111
   Name:    cs764
   Address:  10.107.17.111
   
   ping -a 10.107.17.111
   Pinging CS764.kldev.bluecoat.com [10.107.17.111] with 32 bytes of data:
   Reply from 10.107.17.111: bytes=32 time<1ms TTL=128
3. You can also SSH to the ProxySG appliance and run the command test dns client.ip.address to make sure the appliance is able to resolve client host name.
   #test dns 10.107.17.111
   Performing RDNS lookup for 10.107.17.111
   Official Host Name: cs764
   Host Aliases:
   Cache TTL: 3595, cache HIT
   DNS Resolver Response: Success
4. Create a policy rule to include a Web Access Layer, with a Source to match exactly (make sure it is the exact name) or contain a string (such as "clientA" if reverse DNS resolves clientA.example.com).