I'm researching how to create a custom payment method that outputs an XML file of the order. Is this possible in Magento? Does anyone know any resources to help me? I've read through a fair amount custom payment method tutorials and I haven't found anything that relates to what I need to build.
(then proceed to part 2 and 3 of the tutorial)
These links really do a good job in writing a module to create a custom payment method in magento.
As for creating a custom xml file for the order, I think there are two ways to go about this.
1) Write an observer for when the order completes to do this
2) Create a cron that will run a model that collects all orders you want to process into an xml file. (You may need to create a column in the sales flat order table and sales flat order grid table that holds a flagged value to keep track of whether or not the order has been written to xml)