Silly question....did you enable IPv6 on the ASA interfaces and assign IPv6 addresses?  Have you run a packet capture to see what is happening?  Can you ping the workstations from the ASA?

I have 6rd running on my Linux server through a Comcast Business Internet connection.  The first hop router, however, is a Cisco ASA.  I had to put in specific rules to permit protocol 41 to be passed through the ASA.  Depending on what device is per...