Load custom-data to On-Chip memory

I think using system console would probably be your best bet then if you don't already have a Nios II processor already in your design. System consoles is scriptable so you should be able to take a text file and dump it to any memory location you like. This method will be quicker than recompiling the hardware changing the on-chip memory contents. I think there is a system console example that shows how to read/write an onchip memory so that would be a good starting point.