What's your workflow like to do this?

I don’t do it, because I don’t find it valuable. I’m kinda confused at why patent poster wants to do this. Just pointing out that git supports it too.

There's nothing to it, it's just "git commit".

You then add your changes and "git commit --amend".

(These could be aliases. Mine are "ci" and "ca" respectively.)

You can do that. But I mean there is literally a spot in the .git directory for the work-in-progress commit message. When you later do git commit -a or whatever, it pulls up the message.

Is there any way you can be specific enough for people to reproduce this?

Initial empty commit with just a message you can create with "git commit --allow-empty -m'Early commit message"

Then when you have the changes lined up you --amend that same commit.