Actually, I also remember getting Linux to boot from the harddrive after they removed OtherOS support back in the day - a lot of PS3 hackers took years to do that and then took all the credit cos I didn't announce my work.
I thought it was too obvious.
Basically you just need to set the root device when booting the kernel, you can't decrypt the OtherOS side while GameOS is booted, but you can just overwrite it.
I used a kernel with internal initrd and busybox (cont)