Another trick you can try is to place a notify (I know this works with mIRC) on one or more of the "regs" from that chan. So when they login you are sent a notification that "reg user" just signed in. Then just ask them what the new chan is.

This usually works quickest when you set a notify on the chan bots, course they won't answer but you can do a /whois to find out what chans they are maintaining.

Good luck,
Weapon X