You may not believe it, but sometimes there's legal issues that prohibit such things because you're offering to sell your alcohol at a discount to some, but not all your customers.
The mug clubs I've seen in the past (before the legals issues were issues), all hung their mugs under the overhead cupboards at the bar. Usually, those "members" who were fairly regular also had a place they sat which was fairly regular. Their mugs would be hung over or near their stool.
I went looking for an old thread about mug clubs, but cannot find it. I think it was in the original version of FS-com from back in the 90s/early 00s. Sorry.
Ciao,
Give 'em what they want. Just make it better than they expected.
Operation: Flavor
Get inducted!