Add user note in CiviCRM on behalf membership page
I would do this by using a CiviCRM integrated webform (using the webform CiviCRM integration module) and the CiviCRM contribution page. Use the webform to collect some of the information, including your comments field, and then on submission of that form take the user to the contribution page where they can initiate the transaction. Its less than ideal, but it works.

Work is going on to enable payment processors like Paypal website payments standard to work with webform_civicrm, but it's not ready yet, so I use this approach whilst awaiting that feature to be launched.