It might be worth contacting the event organiser if you cannot book on the platform. We have accepted some on-the-day entries recently. Provided a rider has a current ACU licence it should be possible.
I don't know anyone in our club having any problems as a rider. We have had 50+ entries recently, rather more than normal. There are still a few things to iron out on the other side - putting the events on and working out the money side, but we have no negative feedback on entries.