I'm not sure really - it's a 1 line change to make the "'ajax/forms/new_user.html" something else, so its not a lot of work.

Am I crazy? Does this even matter?  I would defer to you on what would make the most sense here.  I just looked at it and went "woah, that route sticks out like a sore thumb."

Perhaps:
"/new_user.html" or
"/forms/new_user.html"