Implementing a new Data layer is not a trivial task. I think at least in theory an xml data layer could be implemented. The business layer generally wants to consume IDataReader so if there is such a thing as XmlDataReader and it implements IDataReader, then I think its possible.
What you would do to start is create a class library project named mojoPortal.Data.Xml, but then in the project properties make the Assembly name and the default namespace mojoPortal.Data
Now you just have to implement all the static classes and methods by copying the method signatures from one of the other core data layers. You could go by any of them as a guide but probably mojoPortal.Data.MSSQL or mojoPortal.Data.MySql is what I would use as a guide. (Though there may be a few unused methods that you could leave out)
When you can change the reference from mojoPortal.Business to your data layer project and it builds then you have implemented all the methods and are ready to test.
It will be a good bit of work I think, but would be very cool if it was implemented.