http://dtosoftware.wordpress.com/2013/03/13/an-immediate-mode-gui/
As it's almost stand-alone, I'm releasing the source too:
http://code.google.com/p/scavenger-library/
It requires Visual Studio 2012 with the November 2012 compiler and Allegro 5. Only the x64 build is working.
There's other stuff in there too. I'll talk about that when I have time to document it.
I thought about using a IMGUI, but I've read that it often causes more problems than it solves for more complex interactivity. But I will take a look at your code, and I am starting to build a level editor, so it would be interesting to see how the GUI works in yours.