Finally! Apparently there was a show on TV or something where some songs of the show were performed in japanese. They're sung buy the original Cast if I recognize them correctly.
You can find them as usual on YouTube, here:

Something more
http://www.youtube.com/watch?v=lO55c1ajHEU

Only love
http://www.youtube.com/watch?v=Ng1q9E7bIq4 (someone made a photoclip of this one for reasons that surpass me...)

Yes, the songs were definitely sung by Japanese original casts, Yoshio Inoue and Rena Sasamoto, on a TV musical show in June. :lol:

There is also a good news which may interest you.
A musical concert "Frank & Friends" is planned next March in Tokyo.
Many songs from Wildhorn musicals will be sung by Takeshi Kaga, Yoshio Inoue, Rena Sasamoto, and other Japanese musical actors.
Mate Kamaras, Lukas Perman and hopefully Frank Wildhorn himself will join the show!

http://www.grand-arts.com/yi/stage/2010/20...nk&friends.html

Act1 Musical “Mitsuko”
Act2 Frank Wildhorn's Song Book