Since it seems that no one was able to find the boy effect in this version, I took a look at the Japanese 2kki wiki, and found some guide to get it: http://www31.atwiki.jp/sindame/pages/61.html
Accordint to this wiki, the boy effect is located in the new yellow-ish block maze accessible from the geometry world.
The problem is that I've been there before, entered one of those large things with red doors, and it was some kind of puzzle, where I had absolutely no idea what to do. After failed to solve it, I'm no longer able to enter those things, but the effect is located in one of those.
(And to make it more fun, there is some kind of white monster roaming around the maze, which teleports you back to the geometry world if it gets you. And of course, there's a shadow lady as well…)
Can someone check it out?