For whatever reason this was virtually impossible to find. Even the Great Google knew not.
Hoyasoft’s Windows Media Server plugin, VideoGuard, enables a wide range of simple content protection mechanisms for video streams: From basic HTTP authentication to referer URL checking to MD5 ticket validation. VideoGuard’s snap-in admin interface is intuitive and makes setting up URL-level authorization and/or authentication strightforward.
Though the documentation could do with a serious overhaul, Hoyasoft’s email-based technical support is quite good. (Thanks Steve!)
And most importantly, VideoGaurd seems to be the only product on the market offering this kind of simple security functionality. (Which should probably have been built into Windows Media Server in the first place…)