After going through a decompiled version of the extra, i found the password, which i couldn't find anywhere else after playing through the game another time. The password is and most of it is rather unnotable, the 5 portals in the beginning all leading to variations of the miku from lovetrap?, with the path to the right sealed off by a wall of 5 blocks. After some alterations to the wall to make it passable, the portal was sealed off. to the right are screens of more interest.
https://gyazo.com/2e38d0cc7d3ff98d5adc6cc755285d45
https://gyazo.com/34a21bb79e6cac2462c16342b976f570
and the final? screen to the omake
warning: :cirLewd: https://gyazo.com/8a8505f4e7421e17df2e0c4a67d5051e
With all this, i can say my curiosity is sated :atkHappy: