With a center channel, is true 5.1 channel audio possible?

Negative.

5.1 audio requires a special kind of digital datastream which can't be read or decoded by the stereo-CD-player-targeted DSP. I don't even think you could do it with the digital output board because all the 5.1 decoders expect a different samplerate than what our player delivers (48 instead of 44 if I recall correctly).

Not even matrixed Dolby Surround (4 channel: L, R, Center, RearSurround) could be done, because there's no Surround decoder on the DSP chip, and its only input is stereo, so you couldn't do Dolby decoding in software either because there would be no place to send the other two channels.