Hmm, assuming fastboot isn't lying to me, it seems to have worked. I ran 'flashboot flash boot boot.img' (to which fastboot reported OK) and then continued with the boot in the bootloader. It booted, graphics work, along with all other basic functions like wireless and touch input. To confirm, that is with the boot.img that previously did not work with 'fastboot boot', and the one that I pulled from the device itself.
Any ideas why 'fastboot boot' isn't working properly, while 'fastboot flash boot' works? I guess I can test my custom kernel by seeing if the device boots without graphics, but ideally I would like to test the whole system, not just the kernel after booting. Besides, what happens if my kernel change does in fact break the graphics - I would assume that the fastboot broke the graphics, but when it comes to flashing it would still be broken.
I'm not marking this thread as solved until some kind of answer is to be had over this problem, but I will give 'thanks' to you for giving me the courage to properly mess with my phone and come out of the other side unscathed. Having that backup there will certainly give some reassurance in case something does go wrong!