Can't say for sure but if there's any player that can do this then the 656a would be a better bet than most. I've never come across a multichannel MPEG track before but the 656a will play SVCD. You might have to rely on the player's internal decoders though rather than using the Bitstream Optical.Coaxial output.
Perhaps someone who has this player can help more.
Apparently philips players can, (there's a utility used in the process of making the 5.1 MP2 tracks which Philips released), but as you mentioned, if any other player besides Philips has the facility, it'll be the 656A.
If anyone else can help I'd appreciate it.
Also, I'd like to throw this in as a side note; How long before we see a successor to the 656A that can do PAL Progressive? Or am I better of getting one now, and finding a way to hot rod it.