I think it's plausible because there was a real Tashbaan in there. 

I think there definitely would have been a real Charn as part of Aslan's country. To me, there's plenty of credible evidence that points in that direction and none that points the other way.

In the hall of images in Charn, as Digory and Polly look at the faces of the past kings and queens, we're given the impression that the rulers were "nice people" in the early era of Charn's history — that they were kind and wise. Gradually they become more solemn, and then more and more corrupted by evil, until of course they culminate in Jadis, who destroyed everything living thing but herself with a single word. We don't know anything about how and why that world's history unfolded as it did, or how different it might have been if those rulers (and presumably other people too) had made different choices down the centuries. But the implication is clear: Charn didn't start out evil.

More to the point — as we all know, the Chronicles of Narnia are written from a Christian basis. There is only one Creator / God in that worldview. In other words, Charn can only have been created by Aslan. And as we discover in LB, the mortal versions of Narnia and of our world were or are "only a shadow or a copy" of the "real thing" in Aslan's country.

There is no other logical conclusion, then, that the "shadow" version of Charn (which fell into utter corruption and was destroyed) must have its real, and entirely good, original counterpart in Aslan's country. Otherwise, that means there was at least one world that was created on some different basis — that didn't echo some part of Aslan's country — or that was made by some creator other than Aslan. Neither of those options would be acceptable from Lewis's religious standpoint.

Also, there's the scene near the end of MN where Aslan warns the children that their (our) world is "growing more like" Charn and that "It is not certain that some wicked one of your race will not find out a secret as evil as the Deplorable Word and use it to destroy all living things." (Obvious reference to nuclear weapons there.) That, I think, tells us two things:

1. Just because a world may become so overrun with evil that "some wicked one" manages to "destroy all living things", does not mean that that world doesn't have its entirely good and eternal version within Aslan's country. As we know from LB, our world does.

2. When it comes to the thorny questions of fate vs free will, Lewis clearly didn't believe in fate in the absolute sense. Even Aslan can't say for sure what our world's fate will be and whether or not it really will go the same way as Charn. Free will and human choice obviously play some part. And that in turn is more than enough indication to me, at least, that Charn too was not absolutely foreordained to become as thoroughly evil as it did and to end the way it did.

So, no, I don't have any hesitation in imagining Aslan's country as including the real Charn — as it was truly meant to be, without any trace of evil. I very much doubt Jadis ever got there, though, considering not only what she did, but also her attitude towards Aslan himself! 

I am definitely inclined to think there would be a "real" Charn within Aslan's Country - just like the other "real" worlds within and jutting off from Aslan's Country.  I think as @courtenay has given a very good answer as to why - so I'm not going to go too much into detail, but essentially, just like the other worlds in the Narnia books, there were good or "nice people" in the world too.  It seems to me in keeping with the Aslan's nature or (who He is in other worlds) is in keeping with Him accepting what is good and right etc in each world, and what is evil being destroyed and cut off.  Also for me - there is the fact that Aslan has good knowledge of Charn - knowing what happened when Digory & Polly were there and knowing its state etc - suggests that He had some sort of intimate knowledge of the world too and possibly even was there with another name too, just as He was in our world.

@courtenay that's an excellent (and rather profound) response to the question!