AAs noted upthread, the 'no new entrants' stipulation is in the Organisers Handbook; it's a guideline, not a regulation. If it was a regulation it would be a regulation (which is why in the recent past stipulations which the Board and Members would see enforced have been 'promoted' from the Handbook to the Regulations).
As is, it seems a rather pedantic technicality left over from a bygone era. The AUK website - which includes the Calendar listing - is recognised as a primary method of communicating with the membership. As the (re)scheduled events are listed in the Calendar, it follows that all those who are interested in riding the (re)scheduled event or otherwise simply looking for an event to ride that weekend will have the opportunity to become aware of it. Whether they take advantage of that opportunity is up to them. So considerations of 'events being open to all riders' and 'level playing fields' simply don'y apply.
If concerned, there is always the option of joining the event carrying a DIY Perm Brevet (do mail the Org first though), but as Martin says, its messy, needlessly so.