As I noticed in the accepted answer, you can add fields by editing the HTML via firebug / chrome dev tools to add additional fields to the IPN sent by the IPN simulator. This way you can add additional fields that you need (for example, recurring_payment_id) that will be sent from IPN to the URL you specify. After adding additional fields, copy html so you can add it every time you want to use it. I am considering writing extensions for the fat monkey (or something else) that add some javascript to the page, which allows you to use additional types of IPN, for example, necessary for repeated payments.
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