I don't know if it's the best method to handle this but I tried just applying the patch manually with substitute*s.
Whatever point the codebase in the patch is, what's being pulled down by Guix doesn't seem to be there yet as one of the files was entirely missing but adding the file manually didn't seem to disturb anything and everything was able to build alright, for me.
I'm now typing this from Qutebrowser, built with my altered QTwebengine, and I've got proper text rendering (at least, for the sites I first noticed text rendering failing at and sites that had worked properly, to begin with).