Sorry for bumping this old’ish topic. Anyhow I would like to see whether there has been any change in this in the meantime. The iDeal marketshare has become so big it will seriously affect any e-commmerce business that don’t support this gateway:
88% of the Dutch webshops offer iDeal payment as an option
44% of the buyers choose this option during checkout
As far as I know there is noone that implemented it or at least noone that has shared the code with the world. As far as open source cms are concerned currently for Joomla+Virtuemart and Drupal there are iDeal solutions available. Like the OP I would like to see a MODx alternative as well. Be it through Foxycart or some other way.