How might I create a "step-by-step process" content type?
I agree with Craig Hyatt but I thought I'd expand a bit more.

Train the authors to use ordered lists - Like you said it's more effort for you to train users. Your times better spent developing the final product in my opinion.

Organize each process into a book - Why re-invent the wheel? It provides you with the basics of your requirements out of the box. Using this will give you the extra time spent developing something to work on the cons of using this, or better put you get to spend more time adding helpful stuff than building it from the ground up.

Create a content type with a "step" field - You mention that this could be more work for you but it's not a huge amount, using CCK and nodereference this could give you a bit more flexibility compared to the book route.

Custom module - There's no need to do this, the above two points will give you enough freedom to code around drupals inbuilt options.

The clear winner for me would be to create a content type and build around that. You can actually set up a content type to use book functionality as well which is what I'm currently doing for a large project. So in this case you get the best of both worlds.