Followup: I created a Scuba magazine, and added a few hashtags to it. It pulled in a few posts from mastodon, but not as much as I'd expect, since my mastodon account that follows the same hashtags sees a lot more action.
I presume my magazine will only grab toots from federated Mastodon servers, but since I'm not following them directly, I'm not really sure how to force federation with them, or how to tell a bunch of Mastodon servers to send their toots our way.