I thought this as well. I figured a good fix would be a small circle around an enemy (much like Attack Of Opportunity's circle) when you hover an attack icon over them. The circle would have an cone showing their field of view. The point of that would be their direct front, assisting in finding their back. As you recommended, maybe high dex, perception, or scoundrel could spawn a small arrow pointing from their back, getting bolder as you level it.
The problem lies (I think) in the differing stances, affects, idle animations, and mob types. Like I fought a group of humans and had diseased a group of them. The marksmen and mage I could tell where their 'hunching' was still their stance, but the guy with a shield hunched more drastically and I had to make several clicks to find his sweet spot.
I think larger or odd shaped monsters exasperate this. Personally i've tricked myself into considering this a challenge booster- "Rogue, get hit back!" "Shut up! It's a troll with huge fat rolls, he is ALL belly...". Regardless it happens now and again, where lining up a backstab doesn't visually fit. I would enjoy a fix or work around.