Hi Bo9bert ,
The immediate criticism is, why didn't you leave a clear coherent message on their answering machine for them to contact you.
For me , in the absence of a Contract (which is not always the method of handling these things )if they haven't confirmed properly in writing / Email , or given you a definitive "go ahead" from someone you personally know , stating the most basic of conditions, re:- fee ( perhaps no fee ) accommodation , PA requirements ,etc., you haven't got an official Gig.
I think you've been told - " If you fancy coming - do so, and we might find a spot for you depending how the evening goes ".
Inconsiderate Bastards.
Always remember what Sam Goldwyn said , " A verbal contract is not worth the paper it's not written on ".
I hope you find a more worth while way of spending your evening - but I know it's a nagging feeling that you may be missing out on something / putting your name about by performing.
I've suddenly changed my mind - why don't you go, and tell us all what happened ?
Either way - Good luck - but don't get bent out of shape over this, there'll be plenty of decent organisers in the future who will do things properly .
Cheers ,
Betsy