Aside from the debate if this is useful or dangerous, I tried several free RAMDisks for Windows, Gavotte RamDisk, AMD Radeon RAMDisk and SoftPerfect RAM Disk - and none of them works well with PostgreSQL.
Either PostgreSQL refuses to use them at all for datafiles or it seems to work, but at some point in time the server tells you, that there is something wrong with the datafiles on the ramdrive and data corruption bites.
Apparently most ramdrives on Windows provide drive emulations only good enough to support simple file storage.
ImDisk is the first I tried, that seems to work well with PostgreSQL.
It's free, has a GUI and a command line, and the driver is properly signed for x64 Windows.