I have to admit my "mock-up" didn't really conform to what I was aiming for. The "data" doesn't really suggest anything to custom attributes. Of course it would be easier to use the inkscape tools adjust these parameters. But the idea was more so that you get a clear and more integrated "editing" layout which fits with the rest of the software.
In my case it's rather simple. I just use the simple circle objects created with Inkscape, read the matrix and apply it ( most objects in our game are circles). But in the end the level designer can also pick the texture, set various properties for each object etc. It's in the end not for me but for them. The current xml editor in inkscape isn't inviting at all. I want these "level-designers" in my project to work directly from Inkscape rather than to use different applications.
I would like to take it a step further and explain a much "complexer" idea which builds on my original:
What if Inkscape had an "Object GUI layout" creator for objects.
The idea is to create specific GUI layouts for objects ( I.E. level properties, player, enemies objects, whatever ) with specific attributes and properties. Which then you can assign to an object in your workspace. These GUI layouts are stored in a "template" library instead of in your SVG. So that they could be used again for multiple files.
Take a look at the Unity3D interface:

It's just so much more easy if we were to use inkscape as more of unified tool than to rely on other xml editors or the not so inviting xml editor build in.
Imagine creating a layout where you can add sliders with min and max values, boolean types, string/numeral fields, whatever data you need to add. And then assign the object to an instance of the GUI layout. The assigned object automatically gets all the fields in the GUI layout injected into its xml tags/node. Any changes made are directly applied ( instead of hitting the "set" button ). And when you need to adjust, removing/adding fields, it will be updated for ALL objects which use the GUI template.
Now that when I add functionality to my level template, I need to get an instance, adjust the xml by hand and then make sure there are no old instances laying around. My it's a pain in the ass for level designers to do stuff over and over. And yes I could use more of a sophisticated xml application. But I see no reason why I should push my team mates into technical stuff and extra software while it should be handled all in one package. I understand that Inkscape is primarily meant for visuals, however Inkscape has great potential because of the SVG format and it should really expand it.
O well that's just something I would like to see, probably never will....
