Well, this topic came up quite a while back, and there have been extensive discussions on this topic over on gleemax. I think the most solid interpretation, which Mort_Q I think at least hinted at back at the start of this thread, is the "nested resolution" interpretation.
On PHB p269 is the "Making An Attack" box. It says "All attacks follow the same basic process:" and then lists 5 steps.
Step 5 is "Deal damage and apply other effects (page 276)."
Now, we know that many free actions have triggers. Some triggers are explicit, like the wizard utility level 2 spells Shield and Feather Fall. Other free actions have implicit triggers, like Elven Accuracy, but they logically still MUST be triggered (or else we must accept that free actions can always happen in the midst of any other action at any time whatsoever).
Thus the logical construction is that a free action which is triggered has to be resolved at that point in the attack resolution procedure on p269. This isn't exactly news to anyone, the game just plain won't work right unless this is true.
In the case of free actions which trigger on taking some other action, and not explicitly on a specific point in the resolution procedure, like the barbarian's Swift Charge, we thus have an explicitly or implicitly triggered action that needs to be resolved in order to complete the triggering action, that is the triggered actions are pretty obviously nested within the action that triggered them. If no other explicit point exists for this resolution, then step 5 "apply other effects" is the catch-all.
So, in the case of Swift Charge you would trigger it at the start of step 5 "Deal damage" and it would resolve at the end of step 5, "apply other effects", which is before the end of the charge action and thus the "no actions after a charge" rule has not yet come into effect.
Alternately you simply have to believe that free actions are all able to be taken at any point within any other action and effectively the entire debate is moot. Of course in that case you will find there are a whole bunch of other bizarre things that suddenly become allowed, like wardens can mark halfway through their move action, etc. I don't know of any of these side effects that are game breaking, but it seems like it could be a pretty significant rules point and personally I'm not eager to open that can of worms.