I am able to use custom composer key sequence like described here: How can I add a custom compose key sequence?
Using this method needs to export GTK_IM_MODULE="xim". But doing so will disable IBus Anthy input for me.
Is there a way to have the sweetness of both, customized composer key sequences AND IBus Anthy input?
During research I have read that composer key sequences in Gnome are hardcoded in gtk+-2.10.7/gtk/gtkimcontextsimple.c, so editing and recompiling might work, although I do not want to resort to such a method yet...
Edit: I don't need Anthy working with the custom composer keys, basically I want to have two input Methods: Japanese Anthy and EN_UK (with the composer)