A colleague of mine tried to import an existing Mercurial repository today. He entered a SSH URL and gave the username and password...
This didn't work and I remembered that this must be because Bitbucket has no way of supplying the password to SSH (short of posing as a TTY and what not).
So I suggest that you discover this when the form is submitted and give a proper error message.
Additionally: can I enter a local filesystem URL (local on Bitbucket's servers!) and import code from a private repository? Please check this too! :-)