Comment 2 for bug 1025291

Not sure about the choice of gtk+3.0 as the package for this bug, but

    https://help.ubuntu.com/community/ComposeKey/

says that these keymaps are "hard coded into the program in source file [gtkimcontextsimple.c]", and

    http://svn.gnome.org/viewvc/gtk%2B/trunk/gtk/gtk-compose-lookaside.txt?view=markup

also indicates that gtk+ (as opposed to, say, the X.org packages) is the culprit here.