Analogous to turning the universe into paper clips

That's a low bar: it's an intentionally silly example. No one actually thinks we're likely to accidentally create a paperclip maximizer AI anymore than we're likely to accidentally include a "number of paperclips in the world" term in a futarchy metric. But something as clearly negative would be mandatory wireheading to maximize a "human pleasure" term.

A less extreme (and less clearly negative, but also more likely) example would be maximizing GDP. Hanson often uses GDP as an example of something you could include in a futarchy metric. GDP only counts market work, however, which means you can increase GDP by moving tasks from "do them yourself" to "hire someone". For example, if I watch my kid that doesn't count towards GDP, but if I pay you to watch them, and you pay me to do whatever you would otherwise have done during that time, it does.

GDP/person is one of the best metrics for "how is a country doing", often doing much better than explicit attempts to measure things closer to what we care about, but put a big optimizing push behind it and soon all the tiny tasks we do over the course of the day are pressured into market work.

Can you suggest a scenario in which futarchy would fail to prevent the universe from being turned into paperclips?