I am not using LUKS, I am using a custom kernel module that implements a block device that presents to the kernel the decrypted SSD. The kernel module receives the key when it is loaded, then it creates the block device that is eventually mounted as the new root device.

I do not know if LUKS could be used for this, I have not examined it. IIRC, LUKS stores the actual decryption key in the encrypted disk (protected by a passphrase) or in the TPM, like most commercial products for disk encryption, which are methods that I do not approve.