We know that Jamie will be released by Catelyn Stark at some and it's been hinted that it will be this season, we also know that Tywin has spies within the northern ranks, so it's likely that when Jamie escapes, word will get back to him at Harrenhall. Once this has been revealed it is possible that out of a sense of honour Tywin will reveal to Arya he knows who she is and let her escape the castle quietly and peacefully.
Tywins a smart man and he knows enough about his cup barer to put two in two together, he knows she fled Kings Landing by posing as boy with the nights watch recruits, knows she is well educated, a Northerner and strongly suspects her to be highborn, he is also aware of the love she holds for her father who passed away. So it is possible that he either suspects or has deduced that she is a Stark.
It's true that he thinks both Stark girls are in Kings Landing, but it's unlikely that he wouldnt rule out the possibility however small, he is very interested in Arya and this could be the reason why, it's not as if she is going anywhere, she is as much a prisoner as a servant as she would be a highborn hostage.
It's also been noted by several forum members that in an upcoming scene with Arya and Tywin, he does something that changes her mind on him, now we know they have had a few chats so far and Arya still wants to stick a knife through the back of his throat, so it's still to come.
Personally I think it would be a great inclusion, lets be honest the producers have no qualms about changing aspects of the storyline, just look at Theons, their motto seems to be that as long as the end point is the same, changes can be made to better tell a story. If released Arya would still be out in the Riverlands and free to go through the Hell of ASOS, granted we wouldn't get so see her slitting a guards throat, but it's a price I’m willing to pay.
So.....thoughts?







