Here's a question. If modifications can be made to an aircraft provided they are inspected and approved by someone suitably qualified (which they can) ought it not to be a relatively simple thing (and we might hope and expect that the Bongo towbar vendors did, though we know of at least one design that is not fit for purpose) to follow a similar process with a car. It ought not to be hard for an automotive engineer (not a mechanic) to check and do calculations. You're not especially likely to get a helpful answer from a corporate (in Japan) being asked to approve a non standard (as they see it) activity in a 20+yr grey import being used in another country and well outside of manufacturers warranty period. Their concern will be to cover their backs by following the book.
P.S. the 'rotten unsafe old car' thing is a separate debate and applies to old cars generally.