@motorollin:
it probably would require updated intuition/graphics libraries. AROS already does that so maybe AfA (AROS for AmigaOS) could do it. You could try to recompile "kitty" demo from AROS. It shows the AROS mascot in a transparent window that allows you to see through the empty parts. I mean, it shows the image without borders as a non-rectangular window.
Ask Bernd Roesch (the AfA author) about it, I guess that it shouldn't be too difficult to add.
Running AmiNetRadio with a non square skin would be cool :-)