I took this a step further and realized that onMouseDown and onMouseUp should have worked, I just used hmi-event:mousedown instead of hmi-event:down.

More importantly I figured out how to get any paramter back to runtime through the interface.

Let's say we have this parameter

<hmi:paramDef name="example" type="string" two-way="true" />

The next line will influence "example" inside the svghmi but it won't affect the value in runtime 

I would like to thank all of you for making this post and working out an example together. I am only starting to understand the basics on custom controls. I have no idea why they had to make it this hard. I come from beckhoff and since its all web based you could just follow any web tutorial. The way it is done in tia is not intuitive and there is not much information available. 

Heroes like you guys are what make this bearable for me :)