I'm not sure if it posted twice for some reason, but I answered my own question here:
https://forums.modx.com/thread/99657/extra-development----manual-and-example-don-t-work. The Doodles example code had a bug, and needed to be fixed, otherwise it worked fine. I submitted a pull request, but it's likely that it won't ever get pulled in.
However, the documentation shouldn't link to a clearly broken example, or even have the Doodle extra as an example if there are indeed much better ways of doing it. Any of the options @sottwell mentioned could be in the documentation instead of the outdated and broken Doodle example. To help new users there should at least be a working extra example that can be referenced in the docs.