When an appointment is modified, a workflow is triggered that creates an email send with the link to the survey. We would like to be able to pass the appointment id to the posted survey (via the email send). So in the posted survey, you would have a lookup field to the appointment that triggered the workflow. This could be similar to the possibility to link the posted survey to a case.
I already use the method described in http://blog.clickdimensions.com/2013/04/guest-post-send-email-with-custom-entity-data-via-workflow.html to insert the appointment subject and start date in the mail. This way I already have a connection between the email send and the appointment.
If it would be possible to fill a field on the survey with a parameter that is added in the URL, that could also be a solution.