From what I can find, Linux can only chainload another linux kernel. (via kexec) Does this mean that a Linux payload like LinuxBoot cannot be used to boot Windows or another OS, either directly or by chainloading another payload from CBFS?

It's nice that a Linux payload can provide superior flexibility and configurability than UEFI with the added benefit of a battle-hardened environment, but the ability to only boot a Linux OS seems like a pretty significant limitation (if this is indeed the case).