Feel free to mess around with my sandbox for mollie.nl's iDEAL API. Instead of launching your requests at http://www.mollie.nl/xml/ideal, use http://menno.b10m.net/perl/ideal and follow the instructions ;-)
After you've requested the transaction (fetch action), you will be directed to a page on this server. There you can fake the bank process and get the GET request back to your reporturl. Make sure you check back with us and not the real iDEAL API!
I am/this site is in no way affiliated with mollie.nl, iDEAL or any bank.