if it were as simple as building Tianocore with SeaBIOS as the CSM, that would be the default option offered, but unfortunately it's not. The neither Tianocore package (the default CorebootPayloadPkg, nor UefiPayloadPkg) has support for a CSM, like the emulator package (OmvhPkg). I maintain the default CorebootPayloadPkg, and have tried unsuccessfully to integrate SeaBIOS as a CSM, but I've also not put a ton of effort into it. If someone else manages to get it working, I'll gladly take a pull request on github