sounds instead like a universe with a regular and predictable relationship between people's beliefs and events in the world.

Not to mention that at least part of the reality must be sufficiently regular for there to be human-like observers in the first place

We close a feedback loop in which people believe that the universe acts in its own predictable way which is discoverable by science. Which causes the universe to actually be that way. And from then on it becomes unalterable because it no longer cares what anyone thinks. The real problem is that of morals. If the universe can be anything people want, then we had better hurry up and figure out what the best possible world actually is, and then get people to believe it to be that way before we lock it in place as actually being that way.

I know nothing about the story/game you're talking about, but I think this is an important and common misconception, and fanfiction that addressed it directly might be worthwhile.

It shows up in a bunch of places in fiction. Grant Morrison's Invisibles and the Nobilis and Unknown Armies RPGs, I think. A particular bit is that something very much like the present-day real world shows up as a part of the greater reality, setting up the plausible deniability that the world might really be like the postmodern meta-reality described.

I'd love to see stories about a rigorous thinker in such a universe working out ways to exploit its ground rules.

I'm not sure exactly how much this would be a different from the regular Mage. As far as I understand, it's already all about exploiting the meta-reality thing using whatever trickery the mages come up with.

I'm not also so sure how doing the rationalist subversion would work in Mage, since its universe is a lot sophisticated about working in a nonrational way than regular naive fantasy works. You're basically up against an adult postmodernist instead of a child who believes in wizards.