I must first apologize for not reading the complete thread, as it is quite long, and though I enjoy reading the ideas, it would take me until morning to finish, and, regretably, I have classes in the morning.
Here is a real-time description of the situation, taken second by second, upon the start of the round in which the wizard decides to cast the spell...
1st second: The wizard casts the cantrip, taking perhaps the full second or just a portion of it. The archers, if dextreous enough, note this and let loose their arrows.
2nd second: If the archers have low hand-eye coordination, it is more likely they will realize the spell has been cast and let loose their arrows, now. The wizard, at this point, is now preparing his spell, unless the spell requires something additional, and so he is grabbing for his material component.
3rd second: Depending on the distance the archers are from the wizard, the arrows would probably be hitting now. The wizard is disrupted from his acquisition of the component as an arrow strikes him, and he drops it or goes reeling at the pain, or the like. OR the wizard gets an arrow in him while performing the somantic components, and is thereby distracted as he groans in pain.
4th second: If the archers are rather distant from the wizard, it is more probable that the arrows would be hitting now. The wizard has gotten his components out, or is partially through his sommantics, but is now struck by the arrow, and reels at the hit, dropping the component or ceasing his sommantics.
This is over half a round, and if the spell requires an action to cast, then he has already taken more time than he has to cast it. If it takes a full round, then he can't have started the spell after the arrows hit. All of the above, of course, is assuming the caster fails his Concentration check.
What I'm trying to say is that the archers cannot get their arrows off in the split-second that it takes to cast the free-action spell. It's just not physically possible. Therefore, their arrows do not disrupt his casting of the cantrip. They disrupt whatever other action he does, thereafter.
Given the "I wave my hands around like I'm casting a spell" case, it is hard to say. It is not difficult to tell that someone is doing something suspicious that might be able to be interpreted as a spell. However, what qualifies this?
In the Player's Handbook v.3.5, it says that casting Quickened Spells (Free-Action Spells) does not incure an attack of opportunity (Pg. 144).
Also from the PH, it says you can Ready and action with the trigger of someone starting to cast a spell. It does not go into any detail of if the person requires the knowledge of Spellcraft or whatever. (Pg. 160)
Someone on the first page took an exerpt from the SRD that stated that a readied action could only be readied for a move or standard action, and therfore disqualifies Free Actions as initiating action from Readied Actions.
Given the spell is a free action, how can you say they even notice that he is casting a spell? Are they going to shoot at the flick of a wrist? How do they know he isn't just pointing at something, or snapping his fingers for the heck of it? They suddenly see this finger flipping them off, and they are puzzled at where it came from, then realize it was a spell, but note it didn't hurt them. And, even still, whatever this minor action may be, it is not of the complexity of action that casting a spell is (as denoted by the difference between a Free Action taking up no amount of time in a Round, and a Standard Action (the typical time to cast a spell) as taking at least a couple seconds of the six-second Round).
Truthfully, it might be clever, but I would say it doesn't work.