Uneven border radius for Goo.CanvasRect item

I'm trying to apply an uneven border radius to the CanvasRect item but I don't seem to be able to find how. I know the CanvasRect item has radius-x and radius-y options, but those don't fit my need in terms of flexibility.

I'd like the ability create an item with different values for all the 4 borders. I've been trying to use the Cairo.Context through the arc() method, but since I'm not able to create the context from a CanvasItem, but only from the Canvas, I'm not able to apply the style.

Any suggestions or workarounds? Or is this even possible?