I also have many years on a dirtbike so i know where you're coming from, however you should also be able to understand my point here. You know when you ride on a dirtbike, you squeeze it with your knees and aslo you are in a very upright seating postion and the steering is VERY light because of the head bearings and skinny front tire and not to mention they weigh almost nothing. Well a four wheeler is an entirely differnet animal as i'm sure you know, much heavier, yu have to slow down or powerslide in corners, you are always wrenching on the bars which are harder to turn because of the added weight and leverage from the long tierods, etc, also you will notice the seating position is lower and the bars higher on an atv making ir ideal for a thumb throttle, when youre only using your thumb, you dont realize how much you are actually using that right hand to hold on and sterr at the same time, i bought a banshee the one time that had a twist throttle on it and i can ride a dirtbike or an atv with the best of them, but with the twist on the atv, i was always hitting or letting off when i didnt want to just trying to hold on. I men i would slide one way and slide my butt off the seat and hang off the bars and it would be hard to control the throttle and the other the same way, on a dirtbike you really dont hang off the side, you bore stay on top and lean it and it leaves your arms fre to use a twist throttle, one additional thought is that if you over accelerate or are climbing and wheelie too far, on a dirtbike, even if your hand causes the throttle to be pinned, you will still slide off the back and most times it comes out from underneath you. on a quad however your legs tend to get caught on the back fenders plus you have 2 gnarly tires just waiting to grind up your calfs as well as a few undred more lbs to land on top of you, i mean you can flip an atv easilly with any throttle, but i'd rather it be operator error than my hand trying to hold onto a twist throttle...
have some faith in these crazy japs man, they have years and years of experience, quads have thumb throttles for a reason...
you can try it if you want but i can almost guarantee you will see what i'm saying about trying to hold on and throttle at the same time...
whatever you decide, best of luck and happy and safe riding...