Oguri Shun and Maki Horikita both worked on the tv version of Densha Otoko, but didn't have any scenes together. However a little digging around (thanks to the yamaki forums), unearthed this gem:
http://www.tudou.com/programs/view.php?itemID=3687710 It's this weird little short revolving around two Yanki teenagers portrayed by Maki and Shun - and they're both male. Hopefully their hair in Hana-kimi will be miles better.
There's a confession of love, random violence, hilarious strutting and TERRIBLE HAIR.
I think Maki has the ability to pull off the girl dressing as a boy androgynous vibe - it really depends on how they style her hair - but Mizuki is always referred to as the cutest boy in the school, and the drawings are hardly butching her up (look out for part 2, where I do pic by pic comparison).
If they can pull off something like this for Mizuki, I think it'll be a success.
Also, come on. Some of the biggest boy stars in J-entertainment land are only an operation away from actually being biological girls, and that's just for plumbing - their faces are pretty feminine. Androgyny is the new black!
*preemptive strike against any negative posts I'm sure to encounter in forums about "omg, they cast HER?"