Both these mods were mentioned in an autospeed.au article:
Process and device for reducing the ... - Google Patents
Crossflow fans sucking air from the middle of the vacuum behind the car and blowing it into the the boundary layer as it separates.
Motor vehicle with flow-influencing devices to reduce air resistance - US Patent 6378932 Description
A spoiler continually flapping up and down, creating low-energy vortexes that disrupt the high-energy ones.
Has anybody ever seen either of these at work, either full-sized, on a model, or in a computational fluid dynamics simulation? I don't know of any CFD packages that'll allow moving parts like that. Both mods look kinda cool to me, because my TDI Golf's achilles heel is its squared-off hatchback, and these wouldn't add the bulk of a full boat-tail or kammback.