While I've had trouble following #plume users via the plume interface, I've discovered two interesting things about trying to follow plume stuff from #mastodon:
1. Paste a plume user's profile URL into the mastodon search box, and you can follow the user.
2. Plume posts can be in categories. You can paste the URL of a category into the mastodon search box and follow any new posts in that category.
For example, this #tokipona category: https://plume.mastodon.host/~/TokiPonaAConlangAndItsSpeakers