If you compare the descriptions of the old man on the barrel and Noal Charin from Winter's Heart (WH), you discover they are nearly the same.

Barrel Man: "It was a scawny, white-haired fellow lounging nearby in the shade. Mat looked at him questioningly, and he grinned, showing gaps in his teeth. His stooped shoulders and sad weathered face did not fit his fine gray coat." (354,ACoS)
Noal: "The speaker was a stoop-shouldered, white-haired old man with a large hooked nose planted in the middle of a sad face with a bundle slung on his back." (355, WH)

Similarities: Both mention stooped shoulders,white hair, and a sad face.

Barrel Man: "The old man tapped a gnarled finger against the side of his beak of a nose. Both looked to have been broken several times." (354, ACoS)
Noal: "He took care with the gnarled hand the other man put in his. It was all knobs, as if every bone had been broken at the same time and had been healed badly." (356, WH)

Similarities: Both mention gnarled hands and the beak nose description of the barrel man matches with the large, hooked nose in the first quote describing Noal.

Barrel Man: "His stooped shoulders and sad weathered face did not fit his fine gray coat. Despite a bit of lace at his neck, he was the very picture of hard times." (354, ACoS)
Noal: "Noal's coat appeared to be of good quality, though, at least in the dim light, and he did have his lace." (359, WH)

Similarities: Both mention good quality coats with lace.

Barrel Man: "...and he grinned, showing gaps in his teeth." (354, ACoS)
Noal: "...he laughed mirthlessly, showing gaps in his teeth. (356, WH)"

Similarities: Both have gaps in their teeth.

Barrel Man: "His hands no longer possessed the strength or flexibility for swordwork, but the two long knives he had carried for well over thirty years had surprised more than one swordsman."
Noal: "He was sliding a very long dagger into a sheath beneath his coat." (355, WH) and "His broken hands had handled that knife well enough" (362, WH)

Similarities: Both use long knives and know how to use them well.

Also, note that Mat seemed to recognize him ("His weathered face looked familiar, somehow, but Mat could not place him." (355, WH))

With all these similarities combined with Mat seeming to recognize him, it is clear that the Barrel Man and Noal Charin are one and the same. Now, why I think Noal is probably Jain Farstrider:

1. The age matches (Noal is old and Jain should be about 70 now)

2. Barrel man has a messed up mind, or at least memory, consistant with the last sighting of Jain Farstrider.

3. Noal "told stories before going to sleep, stories that Harnan and the other Redarms seemed to swallow whole, ever the one about some place called Shibouya, supposedly beyond the Aiel Waste, where women who could chlenld had tattooed faces, over three hundred crimes carried a penalty of death, and giants lived under the mountains, men taller than Ogier, with thir faces on their bellies. He claimed to have been there." (538, WH) If I remember correctly, The Travels of Jain Farstrider had a similar description of the lands beyond the Aiel Waste and Jain Farstrider is the only person known to have gone there and returned.

4. Noal says "I've seen great men laying plans, and villains dark as the Pit of Doom, too." (539, WH) To have seen these thing, Noal would have to be well traveled and probably an important person to know of great people's plans. Jain Farstrider matches this desciption well.

5. Noal is a mysterious character traveling with Mat, and knowing the series, there is probably something important about him.

6. RJ often adds characters by exposing them to one of the main characters in a seemingly unimportant role before they meet with the main character again (ex. Min, first seen in TEOTW, but not a major character until much later on, Aram, Elayne, etc.). The Barrel Man encounter matches this pattern well, and Noal needs something to make him interesting.

Now that it seem Barrel Man= Noal Charin= Jain Farstrider, what does it mean to the story? Noal may act as Mat's advisor and guide. Jain Farstrider's knowledge of lands beyond the Aiel Waste and Graendal's trying to make Sammael look to the lands beyond the Aiel Waste in Threads Woven of Shadow, LoC suggest that eventually something is going to happen in the lands beyond the Aiel Waste, and it is probably going to involve Mat. Also, Seanchan and Shibouya could be close together (the world is round) and traveling to Shibouya could involve Mat's Seanchan troubles. In addition, if any of the main characters goes to Shibouya, as I think one will, it will probably be Mat, since Perrin is busy with the Shaido and Prophet, Rand is always busy, Elanye is busy governing Andor, Nynaeve is with Rand and generally does not go anywhere by herself, Min wouldn't want to leave Rand in the condition he is in after WH, and Egwene is busy with the power struggle in the White Tower. Mat !
is the only free character, and also will be pursued due to his kidnapping of the Daughter of the Nine Moons.

In summary, I believe the Barrel Man is Noal Charin who is Jain Farstrider. I think he will advise and guide Mat in a trip to the lands beyond the Aiel Waste.