We have built a couple of customisations at Blackbaud Pacific that do exactly this...

The first BBIS custom part allows an existing constituent to RSVP yes or no, to an event invitation via an email link (without needing to re-enter all of their details or pick ticket types, etc.).  The confirmation page and email it generates can include a QR code of the Registrant ID, as the registrations are written back live to CRM so we have that detail at that stage.

A secondary process to provide QR codes via a logged-in user, and/or via an email link landing page in BBIS pulls Registrant IDs out for any upcoming events that people can print out or show on their phone as they arrive.  This is used as part of the event reminder email process too. QR codes can actually be generated through Microsoft Word too if printed copies are to be mailed out to registrants, e.g. for those who have registered and don't have an email address in the system.

The second custom part, acts as a mobile application (but is a BBIS web page).  It allows selection of and display of a list of events via an Event Query, and from there accesses a mobile device's camera (or a laptop with web camera) which scans the QR codes and marks the registrants as attended.  From there you can edit their guests, adding additional 'unnamed' ones, or removing certain ones if desired.  Also supported in the part is a list of the guests (filterable by status) which can be used to quickly find people who are arriving without a QR code... also supports a Constituent Search screen for walk-in registration, can add 'unnamed' walk-in registrants, and includes a live Event Report showing arrival time, percentage of registered vs. attended, and specific reports to identify VIP attendance (based on an optional, configurable constituency code).