Hah! They did (well, except the name was Dismaland not Fuck Disney). The death of the princess was my favourite part.

Tarring contains the original files verbatim, appended together so would be as easy to scan (processingwise) as the original.

Everyone has the zip alogirithm - it would be trivial to decompress the stream and check for the watermark (although processing power would be required).

Encryption would be neccesary, but it wouldn't have to be too clever.