Can I publish a "locked" chapter to try and get into payments? Or is that against the rules somewhere?

Nooooot a clue! But there were some doing shoutouts for their Instagram followers recently. I won’t name their names as I don’t want them spammed with new followers only wanting shoutouts.
If you check on the announcement threads regarding payment programmes and gems etc, you’ll probably find some people who are doing them.
You can also post about it yourself, or even comment in the payment programme threads if you remain on topic.