Slippery Skin, from Exemplars of Evil, allows you to give up an attack of opportunity for the round as an immediate action in response to someone attempting to initiate a grapple against you to substitute the result of an Escape Artist check for your touch AC to resist their attempt. (The caveats being that you need to decide to do this before they make their attack roll and you are stuck with the result of your Escape Artist check even if it is lower than your normal touch AC.)
The Gentle Way Mastery martial arts style feat, from Oriental Adventures also shows promise for your purposes, as it would negate the advantage reach opponents have for grappling and force them to move to you like anyone else, but it requires several prereq feats and would seem to be antithetical to your character design, since you are unable to move at all, not even 5-foot steps, while the feat is in effect.
Benefits: Once per day, you can make yourself unmovable. You automatically win an opposed Strength check when an opponent attempts
to bull rush you. A creature with the improved grab ability must move
into your space to grapple you, since it cannot pull you into its
space. No spell or other effect can force you to move. If you become
frightened or panicked, you suffer the full effect of the fear but do
not run away. You cannot move, even to make a 5-foot step, while this
ability is in effect. This ability lasts for 1 round per level, but
you can end it at any time.
Combat Stability, from Player's Handbook II, offers a +4 bonus to rolls to resist grapple--along with several other combat maneuvers--or a +8 bonus if a 3rd combat form feat is taken. (You would already have two, Combat Stability and the prereq Combat Focus.)
If your goal is to avoid triggering attacks of opportunity that are (or lead to through improved grab) grapples, there are a few options that shouldn't be hard to make use of, since Tumble is a scout class skill.
Swift Tumbler, from Cityscape, is useful in using Tumble to avoid AoO, since clearly the last thing your scout wants to do is suffer a reduction in movement speed in such a situation.
The Duck Underneath tactical maneuver of the Giantbane feat, from Complete Warrior, may or may not be of interest to you as the requirement to pause a round and take a total defense action seems potentially a dealbreaker, but it is on pg. 111 of that book if you wish to examine its potential. (I would quote an excerpt, but I figure this is one you'll need to evaluate the entire feat to be sure if it is of use to you.)
Personally, I'm not entirely certain why Close-Quarters Fighting doesn't help. Yes, I get that you do not currently threaten the creature before it attacks to initiate grapple, but the whole reason initiating a grapple sets off an attack of opportunity is that the attacker is putting their limbs into the target's square in order to accomplish their attack. The example given in that very feat is a dwarf being grappled by an ogre, which is a size large creature with reach. (Yes, I realize it doesn't say that the attack is being made using the ogre's reach. I'm just saying it doesn't say the feat wouldn't work if the orge were using its reach.)
Prefaced by the preceding paragraph, if your DM is also of the mind that a reach creature puts at least part of its body into threatened territory in order to grapple--setting aside any rare circumstance of grappling via a weapon while attacking with reach--then Sidestep, from the Miniatures Handbook, becomes a great option. Since it allows you to once per round make a 5-foot step after making attack of opportunity, the additional five feet may take you out of the reach of the grappling creature and waste its grapple attempt.
You can even do this during movement as the 5-foot step doesn't count as your one allowed 5-foot step per round nor does it count against movement, so if the AoO is set off by an insufficiently high Tumble check to move passed the foe, and they don't have Combat Reflexes, this may allow you to make them waste their only AoO. (In fact, you might intentionally skip Tumbling while running at the extreme edge of their reach if you know you can Sidestep out of their reach and then race through their threatened area without fear with the rest of you movement now that they've used up their AoO for the round.) If you've got at least a 15 Dex and 8 or more ranks in Tumble, you already qualify for this one.
I Hope these help.