Talked with Fycho in game. My two cents anyways.
I think it's better to use "*" to represent characters that can't convert properly. Using a space " " as Fycho suggests, and using * only for representing ☆ seems kind of odd to me. The goal of metadata is to give as much credit as possible to the original source. That's why we see maps being dq'ed even for incorrect capitalization. Using a space pretty much makes it seem like there isn't even a special ≠ or ♥ or × present in the title at all because Players will only see "Honey Come." If we use an asterisk * to represent these characters, it at least acknowledges the fact that there's supposed to be a special character in the title, but due to osu!'s current limitations, cannot be displayed properly.
Basically, if all we do is use a space to represent non ☆ (star) characters, for people looking at the title it'll seem like the special character was never there to begin with. Putting a * at least acknowledges the character's presence even if it cannot be properly displayed.