First off IOS=/=ISO
They are two completely different things. You can only install BootMii as an IOS, it is impossible to install BootMii as an ISO.
Anyway, newer Wiis cannot install BootMii as boot2, and so the option to install it as IOS was given for them so they could still use it. However, it offers NO brick protection/recovery, because if you can access BootMii/IOS, there are other, better ways to fix your bricked Wii. Your only real option for brick protection/recovery is Priiloader, but it is NOT 100% brick proof. It should be able to recover all but the most severe bricks. If the System Menu or System Menu IOS is deleted or corrupted, Priiloader will not help recover. However, it is the best you can get. And no, there is no way to make it that your Wii CAN install BootMii as boot2.