Shocking! :-)

You never answered if you need gigabit connectivity on the LAN for this node or a gigabit connection to the WAN. If the answer is NO and NO then the correct path given your circumstance (assuming you can't/won't pull another cable) is to use the existing Brown pair! What wreaks of a hack more (since people here are all worried about the next tech coming in after you)? Breaking out a CAT5e to get the job done under the circumstances or creating an EG science project in the clients home with RG-6, adaptors, connectors, solder, fancy tags, ughhhh (I'd kill an employee if I happened upon that)? Come on you know the answer to this! :-)