( Spoilers for the episode, and a bit of speculations. )
ETA: Although I'm still slow in watching the episodes, I am on board with this series now. :)
A few toku DVDs I ordered from Amazon.co.jp are here at last. This included Ultraman Mebius & Ultraman Brothers Memorial Boxset, Ultraman Mebius Volume 7, and GARO Special Part 1. Also in the same shipment is the photobook of Shunji Igarashi, which I bought only because it promised a few behind the scenes photos from the Mebius movie. :)
It is strange, but there have been quite a few toku actors whom I got curious about and when I digged, turned out that they had been in the Prince of Tennis Musicals. I know I'm probably missing a few, but I'm just going to list the ones I cared enough to find out. Please note that I do not necessarily follow each actor's career.
As a side note, Mirai Hibino, the human form of Ultraman Mebius, is played by Shunji Igarashi, a member of D-Boys. While Igarashi has never been in tenimyu, most of his fellow D-Boys members have performed in the musicals.