And what about when your commit is part of a 5-patch series, as part of a 3-series project to bring some internal framework code up to scratch in order to start implementing a new feature? Which commit do you stick your design rationale on, and how do you ensure people will see it?

Commit messages are really good for e.g. "patch that due to this bug" or "refactor this so it's decoupled from that feature", but not quite so good for describing arches in development. On the other hand, you can reference an issue in every commit message, and that issue can lead to the design document.