DIY multiboot?

[Der Geist der Maschine]

Well, a Raspberry doesn't have kexec, i can tell you that.  I need to fire up my PBP and look for it.  A Raspberry 3B is my everyday machine lately.  Nope, on the PBP I can't find kexec, by apropos or locate, or in Synaptic.  Should I try github?

kexec(8) is a wrapper around the kexec kernel functions kexec_load (2) and kexec_file_load (2). You already found the man-pages on your machines. As you stated you want to write a C program then that's all you need. That is a great learning experience. kexec (8) is a simple command-line tool saving you a lot of time, though. On debian kexec (8) is part of the kexec-tools package. I don't know about github, but assume you will find the project website, or at least the source code, somewhere in the Internet as well

~$ apt-file search /sbin/kexec
kexec-tools: /sbin/kexec

The possibilities are endless: instead of re-implementing kboot, you can also try to make it or petiteboot work on your machine. You can also check out manawyrm's mysterious twitter link and learn about tsys's bootloader.

But really, you could boot from the standard eMMC then run whatever scripts to boot a partition on the SSD. (seems like)

Yes, if you mean with script a call to kexec (8).