There is a guy called Timothy on here that likes to wear pyjamas at the club but I have never met him. As to the flip flops I have never seen anybody wearing them and I don't know if they are allowed.
As far as I know clothes wise a anything goes. (If it's Ok K for someone to wear just a jockstrap or a leotard the PJs should be alright.
The rules just say that you cannot be barefoot and must have something on your feet. Last time I went in November there was a bloke wearing sliders.