I got it working at last by chaning Arch Linux ARM’s linux-rpi PKGBUILD to make and install a new kernel.
I wouldn’t recommend this method to anyone else who might be looking to do this as I ran into trouble if the package was named differently, meaning this custom kernel is seen as an update to the linux-rpi kernel provided. As a result they can’t be installed simultaneously (and pacman complains that the current kernel is more up to date than the one in the repos). If a version of this custom kernel ever breaks, there will be no second option to boot back to. Perhaps someone more experienced than I can find a way to get aroundt that problem; but for me, getting it to work at all is a win.
Now to spend too much time poorly automating the process for next time.
Thanks for all the help.