Assuming you're using LUKS with device mapper, this reboot did be able to be a plain kexec, and the raw disk key can be placed in a pre-defined location in RAM,

Assuming you're using LUKS with device mapper, this reboot did be able to be a plain kexec, and the raw disk key can be placed in a pre-defined location in RAM, like how the dmesg buffer is something set up to be persistent, for recovering information from right before a crash, even if only via an automated log push daemon.