Long answer, it is not possible to modify boot2 and replace it with bootmii on newer Wiis and here's why.
The Wiis boot process consist of boot0, boot1 and boot2. When you press the power button on the Wii, boot0 is initiated, this bootrom is hardcoded inside the GPU. Boot0 does some things, and verifies boot1 through a hash control, one hash value is programmed inside boot0 and one hash in on the go, meaning boot0 calculates it. This means, if you modified boot1 the hash would be different compared to what boot0 expects. If the hashes match boot1 is launched, it does more things before initiating boot2, boot2 is verified by boot1, and is only launched if the it is signed by nintendo. However early wiis had the trucha bug (string comparison bug) which let things be fake signed yet the wii would still believe the signature to be correct.
This is what bootmii exploited, since bootmii, a substitute for boot2 could be fake signed making boot1 think that bootmii was properly signed and execute it. So why can't you restore the trucha bug in boot1 then? As I said earlier, boot0 has a stored hash value of boot1, and at each start the boot1 located nand is hashed, and if the hashes match, boot0 proceds to execute boot1. If you would reintroduce the trucha bug, you would change the hash value of boot1 and thus boot 0 would not execute it, essentially hardbricking your console.
