I used my 2003 4.7liter [NOT A HEMI ENGINE] Durango to pull a 5000+lb Diesel Landcruiser 2200 miles twice. I did NOT use a flatbed trailer, but my vehicles both had titles and insurance. I had a brake assist, and it was easy.
20-40 miles at 20-45 MPH in a HMMWV pulling another Hummer? I see no major issue IMHO (others obviously disagree).
Maybe not to spec, but given military overdesign, I would feel comfortable driving SLOWLY, or driving with a Readybrake brake assist device. My dad helped design and test US tanks. Our military overdesigns EVERYTHING!!! (That is why the whole titling issue over street worthiness annoys me no end. - but I digress.)
I used the device attached below, called a Readybrake. As you slow it mechanically activates the brakes of the car behind. It worked amazingly well. One of the few purchases I made that worked better than advertised.
http://www.readybrake.com/
http://www.readybrake.com/store/p1/...Receiver_Style_Supplemental_Brake_System.html
Yes, others will disagree. I can speak to the effectiveness of a flat tow of a 5000+lb vehicle with a 2003 4.7 liter Durango over 2200 miles without issue using a Readybrake. The drive was smooth and at 50-60MPH the brake activation allowed comfortable safe deceleration.
YMMV,
T