This is the wagon I have, I no longer use the cover as it became torn and useless. It's larger than a typical Radio Flyer.

These are the product specs.

Weight Capacity: 250 lb
10 in. pneumatic tires
Product Width: 23-1/4 in.
Product Length: 46 in.
Product Height: 40-3/4 in.
Product Weight: 42 lb.
Material: Steel

I feel towing a wagon opposed to a bike trailer can be comparable to towing a trailer with a motor vehicle. Speeding is not advised, as trailer sway can occur, more attention is required, you can't take as sharp of turns, and I've heard several stories of campers and horse trailers that have tipped over, partially due to poor attachment and/or driving.

Seat post hitching is not possible for me personally, as I have a pannier rack that prevents this. I don't think hitching a wagon real high is a good idea anyway. It should be attached lower, though a custom hitch behind the bike may be required.

Have you seen the attachment system used for the tag-along bike trailers? The attachment between the hitch and the trailer allows fairly flexible maneuvering. Via the trailer, only up and down movement is possible, while via the hitch, it's left and right. Put this down lower and put a wagon to it. A bike could tip over and hardly effect the wagon. Again though, custom work might be needed.

So unless you or someone you know is good at doing custom work on a bike, you'd have to be a little desperate to tow a wagon "safely" (Like me). :-P