The first thing that comes to mind is to just state that you do not know the answer, with the implication you dislike this state of affairs, but not phrasing it as a question. This also has the bonus that someone else you didn't expect might chime in with the answer. Basically, provide an opportunity for any of multiple different people to show of by answering, with each individual one having plausible denyability in thinking someone else would respond. If possible, try doing this in a way and place such that there actually ARE more than one person that could have been expected to answer.