I’d like this feature, too. Although rather than creating a “waiting list” I’d rather have a note that they need to contact the volunteer coordinator, and then set that task to require an admin/coordinator to put their name in for them instead of allowing anyone to sign up.
I tried to work around this by putting a fake user named “Vacant Position” in the slots, but then it lists them as filled, and you can’t see them unless you click into the sheet to look at it.
Our organization (a youth symphony) charges tuition, but we have it set up to automatically give a discount based on the number of hours of volunteer time the student’s parents sign up for. Some of the positions require a significant time commitment (and thus get a bigger discount) and we want to make sure the person knows what they’re getting into and is prepared to commit to it rather than just signing up to get the discount.