I think I found the answer, and it seems to defeat my purpose. In order to go from a 32-bit data bus to an 8-bit data bus but still accommodate the 32-bit size of the processor, the nios performs 4 r/w operations where it previously performed 1. So the byte enable corresponds to the portion of the 32 bit master bus that is to be written from/read to using the 8-bit slave data bus. Thus, the output is still 32 bits and I'm back where I started. To solve this, I used awkward C code to convert back and forth between 1 byte and 4 byte values. Is this the best way?