If refusing to process/print Eurion marked things was a legally required feature (which it currently isn't) in printers/image processing software, it could pose quite a conundrum for OSS, if the law also requires that the feature not be easily disabled by an end user. It's hard for me to imagine OSS software with this property. Yet the law could demand it, and society might find it reasonable.
But how would they add the watermark? GIMP I guess...