Thanks @asmecher for your reply!
I’ve indeed seen https://github.com/mcrider/rest, but considering that it only has 4 commits in total, the last one dating more than 3 years back, and an open pull request that’s been sitting there for over a year, I thought that this isn’t the kind of software that I’d invest a lot of trust in.
Alternately, you might have a look at the OAI interface. It’s well maintained, though not designed to be purely RESTful.
Thanks for the hint! What’s the relationship between OAI and OJS?