I've successfully built a 4.6.4 Linux kernel payload for CoreBoot and flashed it onto the top 4 MB of the boot ROM on my Thinkpad x230. The runtime is a dynamic linked busybox with glibc, as well as a copy of the kexec binary from my Ubuntu laptop.
kexec of a xen kernel (multiboot-x86) results in an immediate reboot into the coreboot payload:
https://www.flickr.com/photos/osr/27913702804/lightbox
kexec of a Linux kernel produces the expected result (panic in this case since I didn't pass any additional parameters to find a root fs):
https://www.flickr.com/photos/osr/27914585663/lightbox
This might be a xen problem (triple fault leading to reset vector?) rather than a coreboot issue, however the same Xen hypervisor config works with grub2 on the drive loaded by seabios payload.
There is this old document that describes a "--vmm" option to kexec, but that doesn't seem to exist in the current kexec tree:
https://xenbits.xen.org/docs/4.3-testing/misc/kexec_and_kdump.txt