I guess what you are saying is "AROS has no built-in AlphaBlitting functions but you can get around that limitation by using an OpenMax library" ?
Is OpenMax.library an AmigaOS style shared library?
Why isn't OpenMax library included with AROS?
I'm not saying anything about the blitting capabilities of Aros. I'm just saying that I understand that it runs on an X11 server acting as a window manager, and the accelerated capabilities of the X11 server are quite limited and the X11 server (on the RPi) typically runs in 16bpp anyway due to performance reasons.
Much like on the amiga (where you need to access hardware & software API's to program the cooper, blitter, paula, etc.) on the RPi you'll most likely want to take advantage of the BCM 2835 via the VideoCore API which I was confusing for OpenMax.
http://elinux.org/RPi_VideoCore_APIsAnyway, who says its not included? I'd assume its probably is... if not i'm sure its one "apt-get" away.
The code I linked to just showed trick method of making a 32BIT RGBA window appear on a 16bit X11 desktop.