The fact that Yossy's a private person could have something to do with it. I mean, the only time I've seen Yaguchi blog about Yossy was when it was H!P/work-related (or for birthdays - Nakazawa's, in particular). And Yossy showed up in Nakazawa's blog earlier this year when they went to Hawaii... still work-related. I don't pay too much attention to Yasuda's blog, but haven't seen Yossy mentioned there either. As for the other H!P members, Yossy was in Mai's blog during their practise for the Ongaku Gatas' live... Ayaka... I don't even have her on my blogroll/news feed, so I can't comment on that.
I'm not too surprised that Yossy doesn't have a blog. As bad (?) as this sounds, I don't think she would put the effort into updating it... partially because she just seems like the type of person who would go, "Why would I write about what I just ate...? That's just stupid." Stuff like that.
In terms of h&A fanservice, well... I don't know. Part of me thinks it's just for the sake of marketing, but then part of me thinks that they're both pretty comfortable with the fanservice... I have no idea where I'm going with this... lol