Not all bikes in Farum, and elsewhere at international HPV events, are finished, "industrial- strength" vehicles; nor do they have to be. Some are "mere homebuilts" brazed up to provide the builders/ owners with cheap(ish) transport alternatives; others prototypes, or more or less ambitious  proof of concept  weld-em- ups done to evaluate a particular engineering or other theorem.
    One such project was obviously this bike of an industrious Dane met in Farum, name sadly forgotten, which distinguished itself from the flock by having an integral chain, or at least a main portion thereof. The most important portion in fact; that, which constitutes the greatest "threat" to the cleanliness of rider's trousers. The builder simply realized, that, long as he was going to use a large- section aluminum profile for the frame, he might as well employ it to hide the drivetrain. So he installed a custom bottom bracket with a small within- frame- cavity chainwheel attached symmetrically in the middle of the spindle, and driving another like it at the other end of the frame. Said and done. And very well done at that, too.
    In this fashion all of the BB- to- intermediate- gear chain is kept away from the body, and potentially could be encapsulated even further, to keep it perpetually clean from dust and grit. Knowing how important a clean chain is for the --never enormous-- efficiency of a bike, it is not such a bad prospect at all!
    After the demise of the vaporware Flevo Greenmachine, which --I believe-- was to have such an integral drivetrain, no manufacturer has felt the call to push the envelope of everyday recumbency in this direction. Shame on ye all!