Awards screeners streamed to 20,000 voting members

A film studio streams its new films to awards voters before release. About 20,000 voters each get the same 10 films. Every year a few of these screeners leak to piracy sites within days. The studio wants to know which voter's account each leak came from.

Playback already uses DRM, encryption that stops people copying the files. Leaks still happen: people film the screen or capture the output of a modified player. Pirates also strip anything the app draws on top of the picture. So the mark that names a voter must be in the picture itself, and viewers must not see it. Leaked copies are often re-encoded, cropped or cut to a 15-minute clip.

Each film runs 2 hours and is packaged as 2-second segments at 5 quality levels. Encoding one film at all 5 levels takes about 4 hours on one 64-core server. Video goes out through a CDN that serves cached files and runs no code. About 3,000 voters join in the last week before the vote.

How does each voter get a stream that names them, and how do you read the name back from a leaked clip?