I wouldnt want my ILs coming for a visit around that time either. Be firm about your boundaries and make this about protecting yourself and your child from undue stress in the first few days of life.
I do think you have to be the one who says it to your mother in law, because your DH is in the middle (he shouldnt be, but many men do this) and no matter what he probably wont feel comfortable telling his mother a firm no -- he will compromise in some way or botch the message and she will end up coming, so I think it has to come from you, even if she then perceives you as a b****...or talks bad about you, we all know you arent one. You are right to refuse her at this time if you need to.
Here is what i would probably say --
Say "look MIL, I know you had your heart set on visiting this year, but as it gets closer I am being forced to re-evaluate everything I had planned for the month of August. I now know from my last birth experience how taxing the last days of pregnancy and first few days with the baby are going to be. Given that, I have decided that we are not taking any visitors (this makes it universal -- not about how you dont want her, and even if its a little white lie, its worth saving her feelings)
during August, because we need that time for our small family to bond and adjust. Please understand that we arent trying to keep your grandchildren from you - we just can't have visitors this August, this close to my due date. DH and I would be happy to have you come at any other time after (insert month here), when we can all really visit with you and spend time with you. But August just wont work for us this year. I am sorry if it doesnt work out for your scheduling, but these things do happen and I thank you in advance for understanding. If you cant make some other later date work this year, then we'll have to just plan on it for next year or plan a visit to see you in Alaska when the new baby is ready to travel."
I would rehearse it a hundred times, then I would call her up, or write her an email (Assuming she uses email) and convey this kind of firm message. After you have said it to his mother, Copy your DH on it, or give him the printout of your script, and tell him it is a done deal -- then ask him to support your decision. That way, if she says anything to him he wont be blindsided or caught in the middle and he can just 'blame it' on you (which would probably be easier for him than confronting her and tellign her no you cant come). If she calls you and tries to argue it, just be a broken record and say, thanks for understanding that it is just not going to work to have you this August. At the very least, she will know where you stand. And even if you say all that...she may come anyway, but if you say all that, she may at least be aware enough to get a hotel room and stay out of your way.