Make a check-box [ similar to "limit wait list" ] in the "Create Event" page
if the checkbox is ticked
then, create a "geo-location requirement" field in that event document on FB,
with a value of "yes"
[ NB: make sure the field name is "geo-location requirement" because this is the name that will be used by the person who will query FB for this field during "Event Sign Up" ]
Edit the "Create Event" screen to allow an OPTIONAL geo-location requirement.