I'm trying to figure out how to build coreboot such that I can boot both legacy and UEFI systems.

From what I can tell, most payloads will load either one or the other, but not both (unless Grub2 can do this - unclear to me).

On the other hand, is it possible to use tianocore + SeaBIOS as the primary and secondary payloads, respectively? Or vice versa, I guess. If so, how would one go about setting that up? 

Menuconfig doesn't let me set arbitrary secondary payloads, so I'm guessing I'd have to stuff it in the CBFS manually somehow, yes?