I've had a got at implementing a stripped down version of your GtkModule but have hit a problem. Whilst trying to build hildon-gtk-enty, im_context is now a part of GtkEntryPrivate rather than a GSEAL'd property in GtkEntry. As such I can't access it. Same with x_n_bytes, but I imagine I could just use a gtk_entry_get_text_length call instead.

Edit: I think the fix is going to be storing the mode in the widget itself and translating it to GtkInputHints/Purpose as I thought earlier.