Why don't you try to add the button and AdWhirl view to the same parent view? Something like this:
[view addSubview: adWhirlView];
[view addSubview: button];
It will be same view hierarchy you pursue, in my opinion. You will only need to position the button the way you want.
UPD:
If the AdWhirl fails to receive ads, it fires the delegate method:
-(void)adWhirlDidFailToReceiveAd:(AdWhirlView *)adWhirlView usingBackup:(BOOL)yesOrNo
So you can toggle the visibility of your button using AdWhirl didReceive/didFail methods. Start with adding a hidden button, then on didReceive: set
button.hidden = NO;
and on didFail:
button.hidden = YES;