I'll wait until a forum mod let me publish it. I don't want to make step ahead other projects, as I don't know if this system is already on test, or publish it could make vendors change way to build bios (I don't think they will do it). The way I found doesn't need to use Phoenix Tool Kit, doesn't need to make nothing about hashes or RSA keys. It's really easy and simple. You mod your AMI bios with MMTOOL "like always" to change microcodes or oroms, or any other module (DSTD, ACPI tables, etc), also if you want you can use Phoenix Took Kit to add SLIC, following their guides to make a good SLIC mod for capsule bios, then the way to flash it is where I did the changes. No more security error.
You are in the right path but don't do it that way. The memory ranges at SPI changes between motherboards and even with bios versions so you could brick it, or disable the Intel MEI (it happend to me on the 1st tests). Once a forum mod let me publish about it, I'll do and you'll see it's really easy to flash mod bios without too many risks. Also the Intel FPT version had some changes, wrong one and it will do a bad flash (I did the research with two P8B75M-LX motheboards, and recovering the bios with hot swaps until I found the right way).
Intel B75 chipset uses 5M version of MEI firmware. Common way to update it with 1.5M gives error, force way, bricked motherboard. Asus bios has activated protection from host access, take care