For one even when your loading the ATV never is the full 800lbs at once on the tail gate. You have 2 frt tires at once ,being say 350lbs when loading into truck bed & then the rear tires which has more weight due to the location of engine & rear suspension & bigger rear tires so say 450lbs. I used to use my 94 Ranger for hauling scrap metal & there was times of me loading fireplace inserts & even 3-4 man safes into my truck & my loading ramp was attached to the tailgate & it never failed. The safe was moved using a commercial movers dolly ,So when loading into bed of truck at one point all 1200lbs was actually on the tailgate. It took me & 3 other guys to get it up the 10ft metal & plywood ramp I built.
If all the 800lbs was to be constantly sitting on just the tailgate I wouldn't recommend it ,but being that its just a portion of the weight & its not all the time ,I dont see a problem in it. Do how ever on occasion check the condition of the tailgate cables.
Its just like the car dollies weight issue's. The ones that allow you to move a vehicle around easily in a garage/shop that each tire sits on. Each one is rated at 1000 lbs but say your have them under a 3000lb vehicle only approx. 750 lbs is on each dolly.
Assuming you have a 6ft bed & the rear tires are needing to rest about 6 " onto the tailgate theres not much worry being that the strongest part of the tailgate is the bottom area & braided cables & hardware.