Quoting from http://sites.google.com/site/completesg/hacking-guide/the-basics
"BootMii works by inserting itself into the Wii's boot process, which is as follows:
BOOT0 -> BOOT1 -> BOOT2 -> System menu
BootMii relies on a bug present in boot1 which allows it to be installed into boot2, meaningit can start up before IOS and the System Menu run. Boot1, while a part of the NAND, has its sig hashes stored in OTP (One Time Programmable) Memory and therefore it can never be updated once the console has been manufactured. Consoles made before mid-2008 have the older boot1 and can have BootMii installed to boot2, while consoles that were produced later on have an updated boot1 that will not allow you to install Bootmii as boot2. With these type of Wii's Bootmii can still function if you have a method of booting into the Homebrew Channel. It's not as brick proof as BootMii as boot2, but its better than have no brick protection at all. "
Now, I know that you can boot into the homebrew channel with Priiloader, but I'm not entirely sure how useful this is. In any case, ModMii generates a custom guide for you based on the information you give it, so there really isn't that much that can go wrong.