It's possible to access your service at sparkmuse.appspot.com but Twitter OAuth redirects to sparkmuse.com. If you can configure the authorization so that Twitter redirects to appspot URL it will be possible to register. Then you can do a 301 redirect to sparkmuse.com to prettify URL again.
That seems odd, you can access appspot but not the www domain itself? And you can not access the www domain via a Twitter redirect, but you can access it to get to the login page?
I cannot access www domain in any way. I just intercepted twitter redirect and changed redirect address by hand to appspot domain. It worked but now you're out of invites. Maybe next time...
I tried to create an account using vtunnel, don't know if it worked or not. Twitter name was ocanbascil.