After much trial and error, hours of calls to Palm, and going over the Palm Developer's Guide, I have finally been able to modify the cradle I was using with my 650 to work with the 700p and Centro. Now my cradle has audio out using just the existing Athena connector on the cradle. I added a 3.5mm audio jack so I can use my speakers/headphones while the phone is in the cradle. It's perfect for watching movies or SlingPlayer. HotSync and USB charging still work fine.
I am going to modify some more cradles. PM me if you're interested.