Allow users to edit signup only if signups are open...

Event planners on my site generally close events days or weeks before the event takes place, because they need to know exactly how many registrations they have in advance for planning purposes. Users fairly regularly want to change their registration after signing up, which is fine while signups are open, but not ok after signups close, because the planner has already made commitments based on the information they had when signups were closed.

There is a "edit own signups" permission, but it seems to allow editing of registrations at any time. Would it be possible to allow signup administrators to "lock" registrations, so that they can no longer be edited?

At the moment, I haven't granted the "edit own signups" permission, and all changes must be handled via an email to the event planner, but as users get more comfortable with my site, they seem to be quicker to register, and then more likely to want to edit their registration later, and it's getting to be a problem for the event planners.

I hope this is clear.