I've also been playing around with Lync. I think it would make a great UCA replacement, but i too have been unable to find any solid documentation on how to make it work.
It says this in the UCA 5.1 Admin guide under Optional Third-Party Integrated Applications:
Instant Messaging Clients: UC Advanced includes its own integrated chat component. It
also supports launching the following Instant Messaging clients when they are installed and
running on the user's computer:
• Windows Live Messenger 8.5, 2009, and 2011
• Microsoft Office Communicator 2005, 2007, and Lync 2010
Federation Servers: Federation provides a communication path between a single UC
Server and one or more external IM servers to provide UC Advanced users with presence
and chat features for external IM contacts (see page 29). UC Advanced supports IM and
presence federation with the following servers:
• Microsoft Office Communicator Server (OCS) 2007 R2
• Microsoft Lync 2010 Server
• IBM Lotus Sametime® Server 7.0, 8.0.x, 8.5, 8.5.1, and 8.5.2 and Fix Packs