https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/
The website for SC 7th Rasen contains a number of links to hidden pages. Here's what's been found so far:

Logo on the TOP page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/oluadranoh.php
(anagram of Halo around)
途切れない輪。
永劫続く円環は、世界再生の鍵―

Bird (P-chan?) on the TOP page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/ooliycshnrs.php
(Shiny Colors)
この声が聴こえますか?


DAY1 cast Kiriko's name on the INFORMATION page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/kkiyuokkroiu.php
(Yukoku Kiriko)
継続か終焉か。

DAY1 Mei on the INFORMATION page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/imzimiue.php
(Izumi Mei)
継続か終焉か。

DAY2 Rinze on the INFORMATION page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/mroenniiroz.php
(Morino Rinze)
継続か終焉か。

DAY2 Mikoto on the INFORMATION page:
https://idolmaster-official.jp/live_event/shinycolors_7th_rasen/juo8ex/tmooeiakkat.php
(Aketa Mikoto)
継続か終焉か。


All the idol names lead to identical-looking pages with unique URLs.
Seems like one representative from each unit, so more links may appear as the Enkan tour continues.

No-one has figured out what juo8ex means yet.

Feel free to have a crack at Takayama's code.