Towing is not recommended using the backhoe bucket, but if you want to do it, Perhaps you could weld an attachment to the boom so it will clear the bucket when folded. This would be right close to the swing cylinders but in back of the boom. That would be the strongest point and would be pulling from the boom swivel pin and frame, NOT much chance of warping the boom or dipper stick that way. It should end up being about the same height as a regular tractor drawbar by doing it that way.
You would likely have to be careful when folding up the boom and bucket as it might hook it if you brought everything up prior to folding the bucket.