What would need to happen is that while you're typing you can open up the completion panel, but keep on typing. Escape would banish the panel Up and down would move the selection in the completion list and insert the rest of what you're typing and select it right would banish the list and keep the currently selected completion etc...
Is that at all doable?
You can not update the TextMate editor when running a command or plug-in, in other words typing in TextMate to reduce the list of completion is currently impossible. What you could do is move the typing to an external textfield that would in turn reduce the list of completions. To me personally that is not worth investing time and effort in, since I would imagine that it would be difficult not to disrupt the flow of editing when suddenly your typing is thrown into a different editing context. I think Allan is planning on adding shrink-list-as-you-type-completion in TM2.0.
Joachim Mårtensson