You can really change accessoryView to customContentView in iOS7 (and it seems that in iOS8 as well) UIAlertView
[alertView setValue:customContentView forKey:@"accessoryView"];
Try this code:
UIAlertView *av = [[UIAlertView alloc] initWithTitle:@"TEST" message:@"subview" delegate:nil cancelButtonTitle:@"NO" otherButtonTitles:@"YES", nil];
UIView *v = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 80, 40)];
[av setValue:v forKey:@"accessoryView"];
v.backgroundColor = [UIColor yellowColor];
[av show];
Remember that you need set custom accessoryView before the call [alertView show]
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