Hello Zoran,
I've run quite a lot of test recently and the results are in fact inconclusive... Following is the (long) description of experiments I made recently and in the past.
1. BIOS structure. T400 Bios does not fully looks like T420. It seems that BIOS does not start at 0x500000 more likely at 0x600000 The MAC address can be find 4 or 5 times in the image at: 0x22F48, 0x81FDD, 0x5F6000, 0x5F7000. In fact it is in different places in different images, however last 2 locations 0x5F6000, 0x5F7000 are always the same. Looking around I find nice info about the bios, made by people that by-pass whitelists in lenovo bioses - You might find it interesting:
http://www.endeer.cz/bios.tools/http://www.endeer.cz/bios.tools/bios.htmlhttp://web.dodds.net/~vorlon/wiki/blog/Upgrading_a_ThinkPad_BIOS/With phnxsplit I was able to get 60 different files out of the bios image and it seems that the tool works right. I'm attaching a list of modules the program found, description of "code characters" can be found in phnxfunc.c. This tool compiles on Linux, but it need some simple patching because of tons of compiler warnings.
2. Coreboot/libreboot. For testing I used precompiled libreboot image from
https://libreboot.org/release/stable/20160907/rom/grub/ made for T400. Can be put on any machine (overwritting all flash chip) and it works equally well. MAC adresses are at 0x1000 and 0x2000 in the image and can be changed with ich9gen - I think that You know it well.
3. Moving bioses - this is strange. In the past when I just started working on T400 I had one board with already installed coreboot and one with original bios. Coreboot board had ati and intel graphics, while bios board only intel. I decided to exchange flash chips and it worked. Now it really sounds strange, but both boards booted OK and original bios correctly detected that it is on dual graphics board and show right menu options. Then after upgrading bios to the latest version (3.22) the board experienced long booting problem. It happens and there are threads on lenovo forums about it, so I assume that it nothing to do with the chip exchange. I tried to fix by changing settings of TPM chip and after enabling it the board did not boot at all - I left the board as spare parts supply then.
Now I took it back and started to experiment: put the libreboot image - works right, but any other original bios image and it does not boot.
On the other hand other board (with just intel graphics) works with any original bios image - I've tried 2 different, again overwritting whole chip.
It seems that the problem is not related to flash chip data but maybe to RFID memory You mentioned, or TPM. I don't know what can I do about it - maybe boot the machine with coreboot and then try to change some TPM settings on Linux??
4. Further tests. I put back 2 T400 laptops with easily accessible programming connectors, so now I can play with any images without complicated disassembly. If there is anything I can check/post/try then let mo know. My ultimate dream would be to have tp_smapi functionality in coreboot, but it seems that this is a long way ahead. Anyway I am attaching descriptor (0x0-0x1000) from original bios image.