Seem like you can't do the HostComponent metadata in ActionScript, don't know why.
But why not just do it in MXML?
I know it works smoothly in MXML (not a real requirement either to do in as),
the thing is that inside the framework it is done and somehow it works,
The default skin class for a Spark BorderContainer component.
@playerversion Flash 10
@playerversion AIR 1.5
@productversion Flex 4
public class BorderContainerSkin extends Skin
Cheers mate, you are right. Need to look into 4.5 to see if still hapennin.
In the mean time I can migrate to mxml.