Gyah, I JUST posted in another thread, before I saw this one, heh.
I'm with Hormalakh. I really think making sure you manually stay within a dynamic radius on the battlefield shouldn't be the responsibility of the player... with six different party members, potentially. However, it should be easy to banish that restriction, voluntarily, at the cost of an AoO.
Also, I think an AoO should be guaranteed to hit. Because, simply jogging past a Fighter shouldn't be easy. And happening to get lucky as your opponent still rolls a miss or graze, even with significantly boosted Accuracy, is easy. I mean, you're either taking on that person about to swing a sword at your whole body, or you aren't. If you opt to ignore him, what's stopping him from hitting you? Is it that hard to hit someone who's simply jogging past you without even focusing on you at all?
Finally, I think movement should be slowed while engaged. So that, even if you do decide to take a free hit and jog past, you still don't get to sprint, full-speed, to the enemy's back line, and simply eat some damage as your only cost.
As for options for engagement avoidance, I'm not really sure what to do. It seems like it should be easier, for lack of a better word. Not in the sense that the opportunity to avoid engagement should always be readily available. But that actually performing the act of staying out of that circle should be easier. Basically, some kind of "try and avoid engagement circles" AI/command would be nice, as separate from a "just run straight here" command. It's just silly that you could send someone across the battlefield, accidentally set one foot within someone's engagement radius, and be stickied to that person, when the opportunity to get around them happened to be readily available, but the player simply couldn't easily-enough ensure that their character didn't cross a specific line of pixels. *shrug*. I don't think anything should automatically stop you from entering engagement circles or anything.
It'd be different if engagement circles were always stationary, like a lot of traps. Then, you could simply look at where they are, and walk around them. But, when they're moving along with your character (because opponents are moving about), it becomes a lot more difficult to simply manually avoid them, shy of constantly pausing and updating your movement command until your character gets around the threat.