The conventional wisdom is: Read and follow the instructions in the Owners Manual. If the book says "do not", then don't.
However, sometimes the book says "do not", but there are ways that it can be done safely: Lube pump installation, driveshaft disconnect installation, etc.
I do not know if any such modifications are available for a Prius.
There are comparatively lightweight aluminum trailers available for hauling cars that can not be towed flat or on a dolly. They are, however, quite expensive.
There are many vehicles available that can be towed four down without any expensive modifications. Some Honda models, nearly all Jeep Wranglers, many 4X4 light trucks (if the transfer case has a Neutral position), some front wheel drive manual transmission vehicles, etc.