Dave, I don't think there is any regular Mudcat do now. The only room with a Mudcat name that I know is still opened is the Mudcat Concert room which Sandy now opens for general folk/ blues as well as for concerts.Other room's that I can think of that are "owned" by 'catters are, Alison's (Jiggie) "Celtic Friends", dwditty's "dw and friends", Frank/Francy's(Toledo) Toledo folk and my "Squiggle's Folk and Blues" and some of these are opened very infrequently.
I'm not sure that there is any day when 'catters are most likely to be on-line. Time wise, I tend to see more catters around between midnight and 4 am GMT.
If none of the rooms I have mentioned are open, there is a good chance of finding someone in one of the "Live and Unplugged" rooms of which Neil's Barn Jam is one. Most of these rooms do give some indication that they are live and normally near the top of the list (there are quite a few rooms but maybe only one or two are open at any time and generally speaking, everyone supports the rooms that are open at the time rather than trying to open more rooms).
Jon