How to determine the length of the RBF file

Interesting. This obviously merits further investigation.

The only problem I could potentially see is with regards to the NIOS. When the FPGA uses AS programming and the NIOS II also boots out of the AS device, the NIOS II bootloader uses the length to determine where the end of the bitstream is so it can jump to the end and start grabbing the processor's code. I'd have to look more closely at the bootloader to see if it would tolerate that address being early by a few bytes.

With regards to programming the FPGA, I suppose if someone's code were to absolutely depend on that length variable to determine when they've loaded enough data, then it could also pose a problem.

Jake