I'm not sure why you're asking me, you're the DJGPP maintainer! If you meant it as an exercise, then I'd say: I have no idea where we support it on Windows, where, post-Win 3.x (which we don't support), ".emacs" is a valid filename.
I understood that DJGPP had long filename support too, but maybe that's not universal? In which case, not only "_emacs", but other GNU dotfiles should surely be supported with a leading "_"?
In other words, it looks like a quick hack to support the most commonly-edited Emacs file that would suffer from this problem. I can't tell whether it's still needed, and if it is, it should be extended systematically.