I'm also on Gentoo x86_64 and I'm seeing even less than you. I've configured mplayer to point to libAMBXvBA. It's definitely linked against it.
I have libAMDXvBA.so.1 in /etc/X11/XvMCConfig thanks to eselect and the extensions are showing in the log.
root@symphony # ldd /usr/bin/mplayer | grep XvBA
libAMDXvBA.so.1 => /usr/lib/libAMDXvBA.so.1 (0x00007f25d65c5000
But despite that, I get this when trying mplayer.
root@symphony # grep XVideo /var/log/Xorg.0.log
(II) Loading extension XVideo
(II) Loading extension XVideo-MotionCompensation
I just can't figure it out. Many other people have got it working, which just makes it all the more frustrating! I have a 4670 so it should work.
chewi@symphony % mplayer -vo xvmc 11210_20090228223400.mpg
MPlayer SVN-r29330-4.3.3 (C) 2000-2009 MPlayer Team
Can't open joystick device /dev/input/js0: No such file or directory
Can't init input joystick
TS file format detected.
VIDEO MPEG2(pid=3332) AUDIO MPA(pid=3333) NO SUBS (yet)! PROGRAM N. 1
VIDEO: MPEG2 544x576 (aspect 3) 25.000 fps 10000.0 kbps (1250.0 kbyte/s)
vo_xvmc: X-Video extension 2.2
vo_xvmc: No X-Video MotionCompensation Extension on :0.0
Error opening/initializing the selected video_out (-vo) device.
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
AUDIO: 48000 Hz, 2 ch, s16le, 192.0 kbit/12.50% (ratio: 24000->192000)
Selected audio codec: [mp3] afm: mp3lib (mp3lib MPEG layer-2, layer-3)
AO: [alsa] 48000Hz 2ch s16le (2 bytes per sample)
Video: no video