I guess you'll need to replace kinit, or else include the mount command in your own script and skip the normal mount command. Looking at how something like this is handled in the hooks in archiso or larch might help you.
- [arch-general] initramfs: execute own script AlannY
- Re: [arch-general] initramfs: execute own script Ray Kohler
- Re: [arch-general] initramfs: execute own script Jim Pryor
- Re: [arch-general] initramfs: execute own script Aaron Griffin
- Re: [arch-general] initramfs: execute own scr... Enrico Carlesso
- Re: [arch-general] initramfs: execute own... Marco
- Re: [arch-general] initramfs: execut... Aaron Griffin
- Re: [arch-general] initramfs: execute own script AlannY
- Re: [arch-general] initramfs: execute own scr... Michael Towers
- Re: [arch-general] initramfs: execute own scr... Aaron Griffin