Adding custom fields to Manager, searching on sales invoices tagged “For direct debit payment”, then exporting that would be another approach which may minimize eternal database manipulation required. The difficulty would depend on your banks formating requirements / options. If a csv file option existed, external processing maybe trivial.
Many electronic data transfer protocols are moving towards xml, so maybe there will be the target market to support this in the future readily from within Manager but I’m not aware of any current support other than handcrafting the file as done in these export to csv localisations
For an overview on how difficult localisations are to write see Localisation: GST/VAT worksheet programming guide however your application would probably need to enhancements Lubos mentioned.