The most interesting footage comes from the tape labelled 'Sega Katana Demo TRT: 3:25 4/24/98':
This now-digitised footage shows some concepts for what looks like a Scud Race-esque racing game engine, and several other tech demos which could possibly have gone on to be the inspiration for titles such as Virtual On and Sea Man. Another video detailed at the forum here also shows a collection of adverts and news spots from US television. Why these were archived as 'internal' is a bit of a mystery as these news shows were clearly broadcast, but these discoveries show there's still more to be found when it comes to the Dreamcast.
Thanks to Coolmod for allowing us to share this video.