How to track location of a sign-up form in a block?
have you fully tested your guess?  what you've described should work, AFAIK.

other options would include:

1) modifying the form to have a hidden value, set using drupal_get_path_alias() ...  it would set the value on load, rather than looking it up on submit.

2) a google analytics event tracker attached to the submit button the form.