Umm, it really depends on libglade to support it first (which it doesn't, AFAIK)

I'll mark this as an enhancement for now.

This should be of high priority, now, especially with the release of Gtk 2.12 that has GtkBuilder in it.

yes this depends firstly and foremostly on bug 490678

I'm adjusting the severity back to normal (believe me I wish as 
much as anyone else that marking a high priority would mean 
someone would magically come and do the work, but in our world of
meager resources a bug is either new, or it is fixed).

right now unfortunately I cant afford to volunteer my time to
write these features in my free time, implementing/integrating the UIManager 
(or even most other useful GObjects) I can gauruntee will be a sinch (maybe a 
couple days work since you need to write an action tree editor) provided
that the parser framework is properly reworked as described in bug 490678.

Moving to "enhancement".

Citing Tristan from bug #490678:
"I dont know if Glade will ever have or really need uimanager support."

This can be closed, GtkUIManager was marked as deprecated from 3.10 an onwards.

Safe to say this will never happen.