First to address this tiny issue:

One other minor point - if the order of x and y in the DFDL file is important (as it is in the example), do we need a <dfdl:array storageOrder="firstDimensionChangesFirst"> option? OR can we just list y first and then x?

I thought about using listing order. I decided that people want to list the dimensions in the order they think of them. In this case x first, then y, irrespective of what the storage order is. Then express that representational tweak separately. We could make it a default that lastDimensionChangesFirst then you'd only need to express this attribute if you've listed the dimensions backward, or you've got column-major data. I'm opposed to most default settings for rep properties. I'm with the XML philosophy of being explicit about everything every time. But in this case it seems ok. ...mikeb