If you have BootGuard, you will brick your machine, unless you have the manufacturer signing key. We have the signing key for our laptops, so we are able to sign updates for the four systems with BootGuard: galp2, galp3, lemu7, and lemu8.
As far as I'm aware, there's no particular state involved for you to brick the CPU, but you can easily write a firmware which will prevent boot, or fail some time after boot (In the case of removing critical ME blobs). You could brick your board temporarily, but you're going to be using an external flasher (with a convenient little SOIC clip :- ) anyway, so fixing it is a matter of writing back the old/working image.
This is, of course, assuming you are able to write anything that works to the flash, which you might not be able to thanks to BootGuard.
It's possible that at some point (depending on if it gets open sourced) I'll be grabbing some of your work and trying to wrangle it to work on my desktop. Is this an area where I seriously risk bricking my CPU, or are there safeguards that cause these kinds of instructions to either work or safely fail?