Hi, the concept of ports, attached to the outside bounds of an element allowing connections between elements is a widespread concept. Adding this concept would help generic layouting algorithms taking ports into account (such as KIELER) doing a much better job. Since DI defines abstract subclasses, meta models without ports can simply choose to not use the concept.

The spec currently has no constraint sections. Should we add them? For
example, we might require unowned diagram elements to be diagrams

All the Associations in DC/DG/DI have names except just one:

<packagedElement xmi:type="uml:Association" xmi:id="A_style_canvas" memberEnd="Canvas-packagedStyle A_style_canvas-canvas">
<ownedEnd xmi:type="uml:Property" xmi:id="A_style_canvas-canvas" name="canvas" type="Canvas" association="A_style_canvas"/>
</packagedElement>

Is this intended?

There is a small typo in the DD specification document.
On page 17 we can read:
"namely the Diagram Graphics(DI) package (Clause 9) and the Diagram Interchange(DG) package"

The acronyms in the parenthesis are reversed, it should be Diagram Graphics(DG) and Diagram Interchange(DI).

There are some typos in the attributes description of Color data type, we can read:
"Integer [1] - the red component of the color in the range (0..255)"
for all three components, green and blue included.

The description should reflect the correct color.

The final Association in DG.xmi

<packagedElement xmi:type="uml:Association" xmi:id="A_style_canvas" memberEnd="Canvas-packagedStyle A_style_canvas-canvas">
<ownedEnd xmi:type="uml:Property" xmi:id="A_style_canvas-canvas" name="canvas" type="Canvas" association="A_style_canvas"/>
</packagedElement>

has no name. This is at best poor practice and contrary to the guidance at the end of UML 2.5 2.4.2.