In order to implement this function the whole stack needs patches. From the gss-proxy to the gpm client and the mechglue plugin. So the solution is a set of patches for each component.
As far as I can see, the implementation in http://fedorapeople.org/cgit/gd/public_git/gss-proxy.git/log/?h=master-interposetest does already cover it. The composite name is already set in one of the gp_conv* functions.
When Simo has pushed the changes, it should be fixed.
Metadata Update from @simo: - Issue assigned to gd - Issue set to the milestone: 2012 October