Amazingly what I found was that Naito had been as busy outside of the ring since he first faced Wonjongkam as the Thai had been inside the ring.
TV, radio, advertising, books and most interestingly a single.
Released in 2009 Naito's single, a duet with Kato Natsuki, was used as the theme tune to "Keroro Gunso the Super Movie 4: Gekishin Dragon Warriors".
Although the "Sgt. Frog" (or "Keroro Gunsō") series is beyond me it's apparently a very popular manga in Japan which has also received some attention in the US. From the theme tune it certainly sounds like it could be a fun show and at the time of writing all 5 of the "Super Movie's" are available online with English subtitles.
For those interested in listening to the track, you can watch the opening of the movie-with the song, below, thanks to OceanxPlushie and my buddy @asianboxing.