Thanks for posting. Unfortunately, this (like many other feature requests) is most likely not something I will add anytime soon. It is far from a simple feature and would require quite a bit of additional code modification and additional custom database table fields and database functions. Plus, this is a fairly unique request and probably not something most other users of the plugin would need.
However, couldn’t you just add an additional “waiting list” item to the signup sheet that people could sign-up for after the other positions are filled? Yes, it would require some manual work on your part to notify them (or to sign them up with the admin feature that lets you live search for user and sign them up yourself).
If an automated solution is something you absolutely need, we can always discuss programming a custom add-on plugin for you, but wouldn’t be quick/easy or cheap.