There should be an option, within the event settings, to enable this function. EB would generate a QRCode that should be displayed at the end of speaker's presentation, for example.
Something like: index.php?option=com_eventbooking&view=selfcheckin&eventid=XXXX
To use this link, the user must be logged in and, obviously, previously registered for the event.
Security layers should be thought of to prevent the indiscriminate use of the direct link by registrants who are not present at the event. Something like a (very short) start and end time of the active link, or an encrypted eventid, perhaps.
This feature would be especially necessary when there are 2 or more consecutive events in the same location, where two or more check-ins are required.
Checking in for the first event would be easy. It would be read when the registrants entered.
The problem would be reading the subsequent check-ins. Since all registrants are seated in their seats, it would be necessary to go through the seats to read them. Or even worse, take a break between events, empty the venue, and read the check-in again when they re-entered the room.