Single address Avalon MM slave

Is your problem that it won't allow you to have one slave at address xxx0 and another at xxx4 ?

FWIW memcpy() isn't defined (or required) to use 32bit accesses, nor to copy the data is any particular order. So you are better coding any transfer loop directly in C. Given the Nios instruction set having different 'strides' for the source and destination will have to effect on the code size.

Your slave must also look at the byte enable for the low 8 bits, you shouldn't assume it is set.

I'd also consider using a single Avalon slave and muxing your 32 devices below it. It might save logic (since the optimiser might effect that anyway).