Iirc, boot1 can't be updated, hence BootMii can be installed on every Wii prior to 2009.
If it has boot1 v2 installed (newer Wii's from 2009 onwards) then BootMii can only be installed as in IOS.
Even if boot2 is v4, the bugged boot1 v1 allows BootMii to be installed on boot2 v4.