Well I have verified that it is possible to get PCM 24bit/96kHz playback via USB and an VOB container. DVD-Video does support up to 24bit/96kHz stereo LPCM in the VOB (but most applications/players don't support it).
Using Cirlinca's "DVD-Video/DVD-Audio" authoring software (which supports adding uncompressed audio at 24/96 to DVD-Video) I can playback the 24/96 PCM audio in the VOB. I verified that 24 bit is properly supported and not downsampled to 16 bit using 2 different methods:
(1) did a bit-accurate recording of the coaxial digital output of the BDP-83 (to coaxial digital in of a high-quality sound card) while USB-media playback of a fixed test pattern in the lowest 8 bits of PCM in an VOB and it recorded exactly as the original pattern in the wave.
(2) did a low-level sine wave listening test from the receiver (with BDP-83 connected via HDMI), USB-media playback of PCM test-tones in a VOB. The difference in audio intensity is clear between 8 bits, 7 and 6 bit amplitudes at 1 kHz.
So this proves that the decoding of 24bit/96 PCM audio from VOB via the USB interface is correct. We'll see about streaming these VOBs now ....