Not sure if it is on topic and if it has been mentioned before:
I think Zuzinka sometimes joined the action on AmateurLapdancer and on Picasso's sites (hersweethand etc.) Zuzana Z. I guess they were the girlfriends or wifes of the producers.
PS: there are countless old alsscan scenes where some assistants fists the main girl. The unexpected part is more about how much they show of the assistants. Some loosened up and had their faces shown. But I think at some point they stopped and it just were some other pornstars.
again, not sure if this is on topic but there is quite a lot going on at DSO / drunksexorgy with the lez only girls. For example in Pajama Pussy Party Rosaline Rosa gets a bunch of cumshots on her back. Certainly not planned and her female partner Paula Shy seemed quite surprised and wanted no part in this
In TeenageGroupsex ep11 a woman makes out with the main girl (Monika Thu) while she keep banging the guy. I might be wrong but I think she was the barkeeper in the place they were filming. (Well, she wouldn't be the first barkeeper in Prague to be a pornstar, but it seemed like the others were were genuinely surprised)