Hi Ghost
I would imagine some sort of sanctioning body belt will be at stake (WBO International or something similar), in which case international rules will apply. This means that if Barrera -- or Khan of course -- gets cut from a clash of heads, the bout will go to the scorecards if four rounds have been completed, otherwise the bout will be a no decision.
In contests in the U.K. where no title is at stake -- or if the bout involves just the British and/or British Commonwealth titles -- then British Boxing Board of Control rules will apply. Under these rules, which now seem antiquated, if a boxer is cut from an accidental clash of heads and is deemed unable to continue, he loses the fight, even if he is a mile ahead on points.
Interestingly, when John Simpson fought Paul Truscott last month, Simpson went into the fight after having been cut quite nastily over the eye about six weeks previously against a Kenyan boxer. There were concerns that Simpson's cut would reopen and this was one reason why I leaned towards Truscott. So, what happens? It was Truscott and not Simpson who got cut in a clash of heads (this was a British Commonwealth title bout) and Simpson was declared the winner -- although to be fair to Simpson I think he would have won anyway.
My initial thoughts are that Khan wins this, but his chin is very shaky. Khan was down and VERY wobbly against Willie Limond, who isn't considered any klnd of a puncher, and he went down from a so-so left hook against an extremely battle-worn Michael Gomez. I think that Kramden is right in that this is a winnable fight for Barrera. I believe that Barrera will be coming out to win. If Barrera starts getting punished, he may be looking for an exit at this stage of his career, but in the early rounds I can see him being dangerous. It is an interesting fight and I would expect two-way betting action.