Howard, having a brain fart and blanking on the beginning of a song or verse does not imply lack of preparation or even any general memory deficit.   It can happen with a song you know by heart and have performed dozens of times.    Usually all you need it a trigger (e.g., the first word or phrase of the verse, and having a "cheat sheet" serves that purpose. But having a piece of sheet music or a lyrics/chord sheet can also work.
(When I've learned a new song and have done it a number of times from memory, I find that having a very large print lyrics/chord sheet visible nearby the first few times I perform the song in public actually helps me perform it well and without error, even if I don't even look at it.   It relieves the distraction of not being sure whether I'll blank on a part of the song; just knowing it's there helps me concentrate on what I've committed to memory and on my performance and the audience.)

As Stewart says, just occasional glances at that song sheet can suffice and need not be distracting.   (RUS isn't very good here, because it's not formatted for that sort of use and doesn't make it easy to quickly find the key word or chord you're looking for.)

I agree about RUS - and even bigger print song books - in song circles encouraging people to try to participate without preparation. For one thing, if you're looking at the book/page instead of whoever is leading the song, you can't watch their mouth for cues (e.g., phrasing, pauses in the song) and you may well be singing different lyrics or using different chords because you're following the printed page and not the performer.

So I really do understand why the event organizers "encourage" you not to use RUS but do say it's OK to use a cheat sheet or the like if you need to.   

If you feel too restricted by the post-payment notification of these rules, I'd say you should ask for a refund. But it sounds to me like you could enjoy the music event within these late-notice guidelines.

Genie