Combat Expertise is already very powerful, so though the idea is interesting, allowing this would be extremely unbalancing. There's a big difference between Power Attack and Combat Expertise: Power Attack puts down your chance to land a blow but increases the result if you do succeed, which means that you are still, in the long run, balancing for the attack. But Combat Expertise users often act as human shields, since Combat Expertise puts down the chance to hit while raising the chance to avoid hits--if you didn't plan on hitting in the first place, there's no reason not to full Combat Expertise and fight defensively, and Combat Expertise users pretty much all go for the shield already, so there's no real need to encourage them.
To give perspective: A level 5 human ubertank with 12 Dex will have 25 AC on standard wealth. A crazy-powerful Half-Dragon Orc Barbarian with maxed out Strength (so 30 Strength, and 34 in rage!) and Weapon Focus just to be more ridiculous will have +15 to hit at ECL 4. So he hits this dude a little more than 50% of the time. Combat Expertise and fighting defensively up the ubertank to AC 33 thanks to Tumble, so the insane Barbarian can only hit on an 18 or higher. With the overpowered Shield/Expertise rule, the AC of the ubertank is now 38--unhittable save on a 20 by a margin of 3! That means the Barbarian would need a +5 weapon to even hit on a 19 (I gave him a +1 by default).
