Why is this needed at all? As the decrypted key is in memory before the reboot, can’t it just be written to a know location in memory and have kexec be instructed to read it early on?

You’re assuming a controlled reboot. Mandos was initially created to deal with intermittent power failures. It’s also good for kernel panics.

Oh for sure something is needed for a full start from zero. But the much more common case for a computer with backup power is regular restarts after applying patches that require a reboot. Would be much more pleasant for that to work out of the box with no manual interaction at all.