If i remember correctly the Bag of Rats ruling was codified into the DMG in 4e, but existed as a WotC FAQ answer and sort of 'general piece of info' for years since it was discovered in 3e. WotC actually 'fixed' the initial Whirlwind Attack issue by making it impossible to do more than WWAttack in 1 turn.
It was one of those 'while
technically legal, you have to be stupid to actually do it, and your DM has to be even stupider to let it fly' things.
Finally, this seems to be a 4th edition rule in a 3rd edition game. I believe that the rule exists because it specifically is possible to do this in 3.0 and needed to be fixed. I don't believe using combat expertise without an opponent in range is something that needs to be fixed.
Combat Expertise allows you to sacrifice potential to hit, to get AC instead. You take a disadvantage to gain an advantage.
By attacking an empty square with no enemy in range, you get the advantage (extra AC) but completely negate the disadvantage; because there's no-one to hit. This, to me, feels like cheating a bit, you're taking advantage of the letter of the rules, rather than the intent of the rules.
Basically, you're rules lawyering here. If you really want i can rules lawyer right back at you, but i would advise against it, as i'm better at it than you are.
For example; by default, a Melee Attack can strike at any
opponent within 5 feet.
So unless you have a rule that specifically alters this, you can only make Melee Attacks against an opponent within 5 feet.
The link you provided from the SRD is under the Total Concealment menu, it also mentions attacking opponents with Total Concealment, and striking the square you think said opponent occupies. Ergo it only applies when Total Concealment is in effect, and only when you have an opponent who is totally concealed from you.
Fun fact: By a strict ruling of the RAW, you may only attack
opponents, you can't actually attack objects. The exception is you may Sunder weapons or shields held by an opponent. The only time you may attack anything other than opponents or objects held by an opponent is when using a thrown splash weapon, in which case you may target a grid intersection. It gets a little murky as the exploration section mentions attacking objects being similar to sundering, but by strict RAW, you are never given the ability to attack objects, even though that is the clear intent of the rules.
I assure you, i've actually had this argument before, and it is absolutely not RAW legal to attack empty space for any reason other than you believe there to be a totally concealed creature there.
Fortunately, 3.5 lacks 4e's clarification on hidden and unaware, but there you go.
Now quit the munchkinning and rules lawyering. I don't like clamping down, but i will do so under such circumstances. If you absolutely must have your Combat Expertise AC bonus but can't reach an enemy, ready an action to attack the first enemy to come within weapon range, and apply expertise to that.