How to track mailchimp signup form location
I'm sure there's a simple answer to the above question, but in the end I took a different approach described here: Create custom signup block with mailchimp module?

Because I'm using hook_form_submit to control processing once the form is submitted, I can add the form location (using request_path()) to the merge variables that are passed back to mailchimp. That way, the form is the same on ever page that it appears, but the location is passed back to mailchimp as part of the sign-up process.