I'm getting a number of "Hey, I missed the deadline, but I just thought
of _____", where ____ isn't always a terrible idea. Allowing these in
past the deadline seems like it's inviting more late submissions next
year. And a deadline extension seems similar.
What if we had a room reserved (except during plenary sessions) for
last-minute write-ins on a first-come, first-served basis?
That or schedule a 1-2 hour lightning talks bit on the first day and let the "Hallway Track" handle itself for the rest of the conference.