Interesting question, I don’t know the answer but I have an opinion (if it’s worth anything). First I think it’s pretty easy to get into the jav industry as a male so long as you are Japanese and can perform well. Male talent is always a problem. So I think the statement “I know it’s hard to get your foot in the door” isn’t strictly true.
I think if they used an actor from another Asian country, that would be part of the gimmick of the movie, but “Japanese girl fucks Chinese man” doesn’t sound amazing lol. Plus you can often tell Asian counties people apart, they don’t all look the same .
Basically I think they’d put up with a less talented Japanese guy than put the effort into recruiting an Asian looking guy who can perform, understand Japanese and who has a correct working visa.