bell liberty prince
Yes, there was a GH musical titled Bell Liberty Prince, way back in 2008! Unfortunately, the performance was never released on DVD, so there's no way to know what the musical was like. Shinomiya and Umino are missing from the character profiles, so I assume they weren't included in the musical, but there's no explanation as to why this happened.
As far as the plot goes, the text blurb near the beginning is a generic summary of the GH story, so I assume it was an abridged version of Kazuki's love ending. There are some bromides (photos) of the actors, and some of them show Keita and Kazuki in lovey poses, so it's a fair assumption. The musical only ran for one week, which means not many people saw it... I guess it's lost to history. :(
Anyway, this is a complete set of scans from the official pamphlet that was available for purchase at the venue.