After looking with SMK at the way some characters are rendering in the PDF, I've created a function called hcmc:renderingFixups() which does some character substitution. Specifically, we're replacing i-with-dot-below + combining accent aigu or grave with i-with-accent plus combining dot below; and a similar thing with i-with-short-stroke. This last may not be as pretty as we hope, and there are still outstanding problems with the dot below m (off centre to right) and l (off to the left). We may be able to fix the latter by flipping briefly to another font, but that's very ugly. Still, these are minor issues, and so far CharisSil is working well for us.