Success I think. My brain is totally fired at this point, so if someone can see the problem in this, bellow loudly please!
The Channel set I've authenticated has two MessageAgents attached, one of which is an AuthenticationAgent. I call connect on the second Channelset with the AuthenticationAgent from the first Channelset, and then make the call. It appears to be working correctly! I still need to use BlazeDS 3.0.0.544 because I'm generating duplicate Sessions, but I appear to be getting away with it. Anyone see the problem in this?