I figured out the problem, but not how to fix it.
In acs-subsite/www/admin/groups/new.tcl, the name of the new group is collected from the user in groups.group_name. This value ends up stored in acs_object through some form builder mojo in groups::new (acs-subsite/tcl/group-procs.tc) that I don't fully understand. After that it gets run through convert_to_i18n, but the same mojo does not happen there, and I can't seem to get at the value myself to pass it on to convert_to_i18n. The end result is that my nice group name gets converted to an i18n key with a blank message associated with it.
I'll file a bug. Fortunately, for my purposes I can comment out the call to convert_to_i18n, since I don't need it.
