I think if you get the emu-tools package from here: http://sourceforge.net/projects/emu10k1
you can use it to map the rear channels to the second PCM device. I don't remember the exact syntax, but you want to map front to PCM0 and rear to PCM1.
Actually, the driver that is in the Linux kernel (at least the one that comes with Mandrake) has this set up by default. dsp0 for front, dsp1 for rear and dspw (?!) for center/surround. This is with the kernel OSS compatible driver and _not_ ALSA.
Cheers Soeren