It's much easier to modify an OS4.x CD using an ISO editor to include necessary files for booting off that CD first to get the slb loading and then in the Kickstart part to boot the OS4.x Installer disk...
When I tried to do this it didn't work if you could get it to work then you wouldn't need to take the mini apart making the process much easier.
The version of open firmware you are using should make much difference to you are able install on your mini. However open firmware is buggy and doesn't always work in the correct way.
It might install on other PPC Mac's but it won't install on old world Mac's ( the one's that startup showing the happy mac) You can check if you have an old world mac at www.everymac.com
The slb_v2 is the boot loader it loads the files/drivers that are listed in the kickstart layout file.
When its done it exits the open firmware and loads the amiga kernel.
It will then look on the harddisk for the startup file if it finds one it will it.