Just wanted to say thanks for the help to everyone who answered my questions in the past couple weeks -- I just managed to get a utility working that can patch LOC-fixed LUT values in a compiled bitstream, using data from the prjxray databases. This is a foundational piece of a code arc that will eventually allow me to generate cryptographic keys inside an FPGA and have it commit those keys to its own bitstream without external intervention. 

If you're curious, the code is here: https://github.com/betrusted-io/rom-locate/blob/master/key2bits.py and there's a corresponding script https://github.com/betrusted-io/encrypt-bitstream-python/blob/master/encrypt-bitstream.py which takes the patch and inserts it into a fuse-encrypted bitstream for the full PoC. 

Also, the Xilinx bitstream format is truly mind-bending. Makes me even more impressed with what y'all are doing...