The point I'm making is that "folk venues" in my experience means mostly semi-formal sessions in pubs and so forth, and they come and they go, and change the nights they happen on and so forth.
I very much doubt if the Mudcat has the infrastructure to gather and check that kind of information and keep it up to date. It'd be a lot more complicated than just lists of festivals or established clubs.
It's a very simple thing setting up a website and putting in the information that's on the grapevine locally. A lot easier than running a newsletter.
What might be practical would be to have a list of web resources like this - by country/region/district in with the Mudcat resource, like the list of Mudcatters localities. It probably wouldn't get them all, and would be dependent on people sending in the URL of sites of this kind which they were running, or which they'd come across.