According to http://mumble.sourceforge.net/Overlay the built-in overlay only works on 32bit OpenGL. I think mumble supports a web interface but I don't know if that can be made to work with the UI in PA.
I don't see why not PA is hybrid, it's interface is 32 bit while it's caclulations are 64bit. it uses seperate executables for the interface, a program called CoherentUI, a part of chrome.
@teddythebear explain it perfectly right. Coherent render UI, but when "UI frame" is done it's displayed to you by PA engine as surface on top of other rendered elements.
You're totally correct here, UI rendered to texture and displayed by game engine. Some of configs of this part of engine can be found in /media/shaders/misc.json. Actually it's not something you need I guess. If Mumble actually have HTML UI that can be opened in browser it's mean you can open it in PA UI because it's just Chromium (Google Chromium) with some goodies for integration into game.
mumble overlay which stays on screen in PA: For those of us that use Windowed fullscreen or that have problems with the overlay in game, Stint Calidris has written up a little program to handle the overlay. http://puu.sh/4tnHs (from http://espers.net/2013/10/08/mumble-overlay/ )
