I don't think they'd accept Touhou characters since they belong to ZUN, the Hall of Heroes is for original works only. However, I heard there was a Japanese site out there somewhere that contains a bunch of video game characters, use google.
But yeah, Touhou was one of the first things on my mind when I thought of the editor, but I was more interested in the retro-style Metroidvania gameplay. Mind you, the game is pretty hard and has a lot of "guide dang it" moments...Just like you remember!
I like it, but expect frustration.