The only way you can do this is finding the UIButtons within the UIAlertView subviews:
for (id button in self.alertView.subviews){
if ([button isKindOfClass:[UIButton class]]){
((UIButton *)button).accessibilityLabel = @"your custom text";
}
}
However this is the only way to do it because there is not a public API to access these UIButtons and this is because Apple doesn't want you to access them. Accessing internal views of the UIAlertView class is something which Apple does not allow and it is likely that will make your app rejected during the App Store review process.
If you really need to have UIButtons with a custom accessibilityLabel you should look into designing a custom alert view instead of using the Apple UIAlertView class.