Exchange 2013 CAS proxy for Exchange 2010 mailbox via UAG publishing

Hi - question regarding the double-login when publishing Exchange 2013 CAS (for proxy to ex2010 mailboxes and/or ex2013 mailboxes).

I have already read this post but have a slightly different setup:

http://social.technet.microsoft.com/Forums/en-US/e524c884-3739-4d18-aa0c-5fadae000f25/uag-2010-publishing-exchange-2013-requires-two-logons

Internally we have the ex2013 CAS servers set with basic AND integrated auth types for /owa....but the boxes are serving up the FBA page instead.  This wasn't happening at first.  Not sure what changed.

We want to be able to use integrated authentication to these new servers on the inside network, and then use forms-based at UAG for external access....but without the double hop.

Is this possible?  Thanks in advance.