LOL. Isn't there some kind of extra stirrup thing you can use for that? I wouldn't try relying on the landscape myself! Unless you're jumping competitively a tall horse is a disadvantage IMHO.
I'm just not into mounting from the ground. I feel like pulling on one side of the saddle that much has to be uncomfortable for my horse's back. I live in Connecticut, so there's always a rock or something around to do the job. If nothing natural comes along, a car bumper will do just fine. I love my tall horse, although I must admit most trails seem to be trimmed for horses in the 15 hand range. I've become very skilled and doing the limbo under those low branches. My horse is a doll and seems to know to go under the higher side so mommy can fit under there too.