I never knew this existed. Those crazy Japanese wanting their porn on the go. lol
I'm guessing they are unlicensed can't see Sony okaying this. lol
I am not an Admin but my opinion is yes you should be able too, but if you think any show too much put a censor bar on the naughty parts maybe.
The only reason I'd say it this is there are several adult video games on this site so I personally don't see why adult UMD's would be an issue. Just be sure to catalog them under UMD's (JP).
Again
I'm not an Admin but just from what I've seen on this site I'm guessing it would be okay as long as its nothing showing too much.