We mostly send boot options to our Avaya VoIP phones via DHCP options. However, in a few specific cases, it would be easy to send specifc data to the phones based on the access switch they are connected to, rather than via the DHCP pool they are in.
Juniper switches only support 'standard' TLV, and do not [yet?] support the setting and sending of proprietary LLDP TLVs. From what I understand only the latest model Avaya (now Extreme) switches actually have this support, and no current Cisco switches have this capability as well.