Good luck with that Dick- they asked me once with the same restriction- I said no & never heard from them again.
Maybe you conform with the rules as a kind of challenge?
Did you ever get any justification for the rule?- it seems a serious restriction on you and me too & not really a reflection of what we actually do?
Mind you, I used to attend the Sligo singers session, which had the same restriction. I wasn't a guest so no problem in conforming for the 2/3 songs everyone got. One Christmas, it was decided that instruments were allowed, so I took my box & started a song with it... a very out-of-tune guitar then accompanied my efforts of & did so for everyone, regardless- it was awful.
The organiser, the late Frank Finn told me later that a similar previous event was the reason for the no instruments policy!