If it is not strictly possible, is there a standard practice for integrating an external element into an approval process/workflow?
You could do this using some Apex, but it's not possible using standard functionality. Using code you could send an approval notification to the external approval and then check incoming mails for an 'Approve / Reject' mail (maybe include record ID in the body of the email for uniqueness) and then update the record to be approved.
Submitted as answer as suggested.
Equally you could just create some approver users with the email address of some external users. (Or possibly an approver mailbox) They will receive an email notification of approval and can respond to this email with rejection or approval.