We consulted with very trusted knowledgable mechanics before towing the Mini. According to those sources, You can safely 4-down tow a manual Mini only. You won't find any info in the owner's manual. It is not there because Mini has not approved it for 4-down towing. Many manufacturers simply don't bother to approve their products for towing simply because it only appeals to a tiny fraction of potential buyers. That does not mean it cannot be towed safely 4-down. It can, and there are many cars that are not "manufacturer-approved" for that purpose. The best source is Remco (Google that and you'll find the link). They have an application guide, and you will find that the manual Mini can be towed, "as is." One final consideration is whether the car is under warranty or not. Since Mini does not approve it for 4-down towing, there is a possibility that a dealer could question certain types of warranty issues (e.g. wheel bearings, transmission, front suspension, etc.) While there's no reason to believe that 4-down towing would have caused damage to those systems, a dealer or Mini might claim the damage was related. A potential warranty claim is the only real advantage of having a "manufacturer-approved" vehicle for 4-down towing.
After towing for three years, our mechanic said it was doing great.