Well it turns out that this plugin is sadly not compatible with SBT 0.13, so it's back to the drawing board when publishing a project based on the latest SBT.
Luckily, all is not lost. Hinted at in a CloudBees post, your repositories are available via WebDAV at exactly the same location you use in your build.sbt to access them, but via https.
And the MacOS Finder can mount such a beast automagically via the Connect To Server (Command-K) dialog - supply your account name (i.e. the word in the top-right of your CloudBees Grand Central window) rather than your email address, and boing, you've got a new filesystem mounted, viz:
The only thing the WebDAV plugin actually did was create a new directory (e.g. 0.6) on-demand - so if you simply create the appropriately-named "folder" via the MacOS Finder, a subsequent, completely-standard SBT publish will work just fine.
You might even want to create a whole bunch of these empty directories (e.g. 0.7, 0.8, 0.9) while you're in there, so you don't get caught out if you decide to publish on a whim from somewhere else.
