GWG, Some random thoughts:

Your challenge question is tough, not just for the mere discovery portion, but for the multiple other functions involved, particularly a "submit/reply" portion and a separate "I want to subscribe to something for future updates".

I can't think of any sites that do both of these functionalities at the same time. They're almost always a two step process, and quite often, after the submission part, few people ever revisit the original challenge to see further updates and follow along. The lack of an easy subscribe function is the downfall of the second part. A system that allowed one to do both a cross-site submit/subscribe simultaneously would be ideal UI, but that seems a harder problem, especially as subscribe isn't well implemented in IndieWeb spaces with a one click and done set up.

Silo based spaces where you're subscribed to the people who might also participate might drip feed you some responses, but I don't think that even micro.blog has something that you could use to follow the daily photo challenges by does it?

Other examples: https://daily.ds106.us/ is a good example of a sort of /planet that does regular challenges and has a back end that aggregates responses (usually from Twitter). I imagine that people are subscribed to the main feed of the daily challenges, but I don't imagine that many are subscribed to the comments feed (is there even one?)

Maxwell's Sith Lord Challenge is one of the few I've seen in the personal site space that has aggregated responses at https://www.maxwelljoslyn.com/sithlordchallenge. I don't think it has an easy way to subscribe to the responses though an h-feed of responses on the page might work in a reader? Maybe he's got some thoughts about how this worked out.

Ongoing challenges, like a 30 day photography challenge for example, are even harder because they're an ongoing one that either requires a central repository to collect, curate, and display them (indieweb.xyz, or a similar planet) or require something that can collect one or more of a variety of submitted feeds and then display them or allow a feed(s) of them. I've seen something like this before with http://connectedcourses.net/ in the education space using RSS, but it took some time to not only set it up but to get people's sites to work with it. (It was manual and it definitely hurt as I recall.)

I don't think of it as a challenge, but I often submit to the IndieWeb sub on indieweb.xyz and I'm also subscribed to its output as well. In this case it works as an example since this is one of its primary functions. It's not framed as a challenge, though it certainly could be. Here one could suggest that participants tag their posts with a particular hashtag for tracking, but in IndieWeb space they'd be "tagging" their posts with the planet's particular post URL and either manually or automatically pinging the Webmention endpoint.

Another option that could help implement some fun in the system is to salmention all the prior submissions on each submission as an update mechanism, but one would need to have a way to unsubscribe to this as it could be(come) a spam vector.