If a build uses a UNC path as the 'Use custom workspace' (under 'Advanced Project Options'), the path is not recognized as an absolute path, but instead the path is appended into the default 'Workspace Root Directory' (master) or to the 'Remote FS root' (slave).

Thijs Brobbel has emailed a patch fixing this problem, see patch in the link. I have also attached Thijs's patch.

With the patch it's possible to use paths like \\hostname\sharename\path\to\workspace as the custom workspace.

The use case is to allow parallel downstream builds on multiple slaves on Windows, without having to copy workspace data back and forth.