I've just committed one-pass vertical stretching to branch jneeman. I haven't pushed to make web because I haven't yet checked regressions and there is a change in behaviour -- it now stretches systems by default. Stretching can be tuned by tweaking VerticalAlignment #'max-stretch. If you want to tweak the systems individually, you should set VerticalAlignment #'max-stretch to zero and use line-break-system-details as before.
What do people think? Should we stretch by default?