How to track sign-up location mailchimp?

I'd like to capture the sign-up form path used when a user subscribes. What's the best way to do it?

One idea:

1. In Mailchimp: Create a hidden merge field to track signup path path.

2. In Drupal: add the hidden field to a signup form.

3. Use a form alter with something like request_path() to populate hidden field.

However, I can see the Mailchimp module has evolved considerably since I last looked (nice work!) so I want to make sure I'm not missing something more elegant.