Thanks to the buildbot page, we can see which features and fixes are already incorporated in MkvToolnix and whether the respective version has been built successfully. But I haven’t found out yet if (and how) I can download a certain build.
Those builds are indeed available. I call them “continuous builds” instead of “nightly” as they aren’t timed to occur daily, but only after commits to the repository. They’re available here for Windows & here for Linux.
I already was at that place, but failed to actually click on “85.0” in the navigation at the left, because I assumed that the fix I’m interested in would already belong to version 86. Silly me