I like to sketch a proposal for making an opt-in API to allow game clients to render the overlay without requiring DLL injection or hooking Direct3D APIs. This would reduce some crashes, and allow games to skin the UI for Mumble appropriately for the game content. Sorry, but I am sketching this proposed feature without knowledge of the precise techniques Mumble uses to generate the overlay, or of Mumble's overall code architecture.
The client software would check for the existence of Mumble in the process address space with GetModuleHandle, and then retrieve one or more entry points for the opt-in API with GetProcAddress. Calling these procedures would disable the automatic overlay, allow polling of the state of the UI features to display and the user list.