I think the nearest-term most generalizable form of this feature would be to store an optional extra "custom_question" per-event, which would show up on the RSVP form and save its data as "custom_question_answer", which would then be displayed somehow on the organizer dashboard.
I'm imagining something like this:
event form:
rsvp form:
but really the design could be anything so who knows!!
Someday it would be nice to allow organizers a way to collect whole lists of extra custom data but that's pie in the sky future technology stuff.