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.

I suppose really the question about flip flops depends on the aesthetic aspect of feet. Some men's feet are a bit off putting and some are attractive. Just don't wear flip flops if your feet belong in the former category.

Regarding a sub wearing pyjamas is a real turn on for me as when I was at a rather strict residential school in the 60's we were often caned in our pyjamas for being out of bed after lights out. Of course, being out of bed, we were doing somewhat horny boys do. I will leave that to the reader's imagination.