After 24 Hours, Escalate The Question
A simple rule, if a community question goes unanswered for 24 hours (or 48 hours in smaller communities) it should be escalated.
Why not set-up a simple mailing list which all superusers are subscribed to. Any questions which are not answered within 24/48 hours are pushed to the list (in a digest) for your expert members to answer.
If they’re still not answered within 48 hours, have another list which escalates the question to customer support (because some questions require trained staff with access to a user’s account to answer).
If you’re going to take the monumental effort of creating a community where members can ask questions, take the significantly smaller effort of designing a system to guarantee they get answers to those questions.