Intel did a prototype of a multiprocessor UEFI application that would start up cores and UEFI itself does support synchronization on the assumption that applications/bootloaders will start other cores before calling ExitBootServices. However, there are no protocols as of 2.10 (the current spec[1]) that I can find that would bring up another processor. That said searching it can be a bit arcane.
Interesting, very interesting. Curious that the PI spec doesn't include update dates on the versions. The MP spec was introduced in 1.5 and I have no idea when that was released.
