One possible way to approach sequencing in GURPS might be by messing with the rules for Basic Speed.  One could also calculate a Mental Speed and Physcial speed, or some such.  Actions might be declared in reverse order of Mental Speed--so the slow thinkers declare first, and the fast thinkers get to react accordingly.

Then, resolve the actions in order of whichever speed number is appropriate and highest.  Multiple actions divide speed, so if you are doing two attacks, one of which is a Rapid Strike (three total actions), you might act at Physical speed, X, 2/3X, and 1/3X.

As a verbal example, two PCs (Able and Baker) are fighting three cannon fodder bad guys (C1, C2, C3).  Able has DX13, IQ11, Basic Speed 6.  Baker has DX10, Combat Reflexes, IQ13, Basic Speed 5.75.  Cannon fodder have all stats 10, basic speeds of 5.

For declaration, C1, C2, and C3 have to go first, the GM declaring that two attack Baker (one regular attack, one AoA double), and one attacks Able.  Baker then declares a move and attack, trying to ensure both guys can't get him at once.  Wanting to try and be ready to come to his buddy's aid, Able then declares an AoA--Feint and Attack-- on C1, who's coming for him.

We resolve this in order of speed.  Able's Feint and Attack might be a Mental Speed (say, based on IQ11) followed by a physical attack-which sequences at half (two actions) his DX, or 6.5.  Baker will move and attack, both physical.  He squences at his DX and DX/2, but possibly with say a +1 bonus for the combat reflexes.  Baker sequences at 11 and 5.5.  

The bad guys perform simultaneous actions at 10, with one more holdout at 5 for the second of the AoA.

We're left with:

Able feints at C1 and Baker moves at sequence 11

C1, C2, and C3 all attack at sequence 10.  Note that C1 in this case is being feinted AND he's doing an AoA. some of these attacks from C2 and C3 on Baker might not be in range anymore--tough tooties for the slow thinkers.

Able performs the second half of his AoA--the attack, on C1

Baker performs the attack portion of his move-and-attack 

If C1 still lives, he performs the second attack in his AoA.

This moves GURPS to an expicit round system.  It also means something like if you have a character with multiple shots from a firearm, that the old 3e mechanic of resolving each shot separately makes more sense, but the new 4e mechanic is less book-keeping intensive and I like it.  So that kind of action, which should rightly be distributed throughout the turn, is problematic.

 Anyway, good thoughts from you regarding timing.  I've mused on this myself.