Do you have a SP3 with the new USB card? By the sounds of things the answer is probably yes unless that is truly a difference between the BDP-2 a and BDP-3.
Not sure where you are getting the impression that the BDP-2 (or 3 for that matter) will work for outputting multi-channel through their USB ports. Nothing here indicates it, that I can see.
I appreciate that the USB upgrade for the SP3 gets that end of the chain working. What evidence do you have that the BDP (any version) end of the chain works?
I output multi-channel DTS through the BNC connector of my BDP-2 to my SP3 all the time, but it is from compressed DTS-CD files (2 channel encoded 44.1-16 AIF files). Even that sounds pretty amazing at times.
I would love to stream full resolution multichannel LPCM from my BDP. So far I haven't had anyone from Bryston give a complete response as to whether this is possible or not. Chris once told me that my old BDP-1 would only output 2 channel, but it was not a really clear response to the question I asked him. I have tried playing back full resolution multi-channel files through the BDP's BNC (and AES-EBU) and I get the results reported above, only the two front channels seem to go through. That makes sense to me due to the limited bandwidth of the SPDIF connection.
Will the USB output on a BDP-2 handle multi-channel output of uncompressed LPCM? If so I will get the SP-3 upgrade to accept it from my BDP-2.