access epcs as mtd to program FPGA

Depends how you want to do it. If you are using a CPU and you're only dealing with 8 bits then you can just make a LUT in memory using an array where 0x01 would become 0x80 and so forth.

In my project I read a buffer size from the .rbf file on a USB drive. I then reverse the bytes in the buffer and then write the buffer to the EPCS device. I then call the remote update function, which loads the new FPGA image into the FPGA.

If this is not what you want to do, then you need to be a bit more specific in what exactly you are trying to accomplish.