DivX and XviD both conform to 'Part-2' of the
MPEG-4 ISO 14496 standard.
If you haven't done-so already...
Try encoding some 'MPEG-4 (SP or ASP) video with MP3 audio muxed within the .avi container' samples. And transfer them onto a CD-R or DVD-R disc...
It's possible to generate MPEG-4 SP (Simple Profile) or ASP (Advanced Simple Profile) video using either DivX or XviD (and a few other) video codec software's - but whatever you do, do not enable their Qpel and/or GMC options.
Another useful tool is Moitah's
MPEG4 Modifier, which can 'modify' certain attributes of MPEG-4 .AVI contained files, making them compatible with hardware playback devices - without having to re-encode them.
Cheers
EDIT: For anyone who's interested... The Onkyo BD-SP809 is fitted with a Marvell Qdeo media processing chip-set, which technically offers support for decoding: MPEG-2, MPEG-4 SP/ASP, MPEG-4 AVC and VC-1 in both standard and high definition PS (program) and TS (transport) streams.