The signatures are there because I had them anyway for the "chip detector" sketch. May as well say it is chip X, but we can't put a bootloader on it, rather than "I don't know what this chip is".
Are you going to include the host-side program to produce C data structures from .hex files?
Yes, that's documented here:
It's a tiny bit fiddly, I did it with a Lua script running inside my MUD game client. You could make it stand-alone, but the client environment had a file picker, MD5 sum, bit manipulation, and other stuff like that which isn't core Lua. The page explains how to set it up, the stuff is all free, no problems with that.
Basically it reads the .hex file and stuffs the hex bytes into a memory image (to cater for the occasional gaps where there are FFs). Then it sumchecks the image. It also allows for the high-order address byte if you are over 65535.