Nobody is "hooking their Android device to an external media player" for this. They reference using the app MX Player to do the hardware acceleration. MX Player is "external" to the XBMC app, not the ...
Current unofficial builds of XBMC for Android have no problem playing MKV files. As long as the video and audio tracks are in codecs the Tegra 3 supports they are good to go. Unless the Ouya version ...