Yes i tried, i had the same your ideas, then i even disconnected the battery.
After my experiments it seems PS3 use Efuse technology by IBM, if so it should change the encryption key every time it change firmware version, then it should be impossible to restore an old dump (because of key change).
According to what I have read EFUSE allows for reconfiguration of circuitry of the chip - i.e fine-tune the logic or fix some bugs (think of Pentium DIV bug). Using EFUSE to encrypt some stuff is in my opinion far-fetched and would be extremely expensive to achieve something for which there are better alternatives that are proven already (read: cryptography). Also according to http://en.wikipedia.org/wiki/EFUSE EFUSE is employed in Cell processors in PS3. So do not let your imagination run too wild...
I just wonder why they didn't burn a fuse then when they fixed the RSX-Bug.
Have you verified this? Have you successfully went from 2.10 to 2.01 from a dump?
I have dumped every fw from 1.32 to 2.10 on which i currently stand.
I have some ideas that i will be trying but not untill i have aquired a power supply because every idea has a high probability of causing a brick if not successful.
Hardstore's video claimed to successfully downgrade from 1.60 to 1.50 without a problem. After reading a little (because i dont have one) it seems the 360 blows a efuse with every update, and who is to say for sure that whoever wrote that on the wiki is correct?
And the fact that NDT's downgrade test got as far as the ps3 accessing the HDD before failing to blinking red also makes me question the efuse theory. It would seem to me that by the time the ps3 has accessed the hdd that the nand has already been accessed and found ok by what ever is decrypting it. (if you connect the usb of infectus to pc and start the programmer before powering on the ps3 the ps3 will fail at reading the nand and blink red before the hdd is ever accessed)
Who knows though... more testing needs to be done.
You say it's used in PS3 Cell then that my imagination run too wild ...while i stated the same :P
Now that I read your original post and my reply I don't know what I was replying to (especially the part about the cryptography) However my feeling is that to check a version of firmware in FLASH against some hidden constant somewhere, use of the EFUSE is still bit of a science fiction. But I may be totally. However my feeling is there may be some other (small) flash somewhere or something similar that would allow storing the version number. EFUSE use just doesn't feel proabable to me - but it i only my feeling.
Since I've get the feeling that I lack some information I'd like to ask which downgrades/Firmwaremods of the PS3 Firmware (Retail/Test/Debug) are confirmed to work (or not to work):
via Infectus with an previous retail PS3 hot dump: 1.6 to 1.5 (the youtube video from hardstore)
via PS3 Retail to Debug Firmware HDD swap trick:
<1.80 to 1.80debug partial
<2.01 to 2.15debug partial (see XVISTAMAN2005s post)
I also lack infromation about the cold dumps... what do you need to do them (I read somwhere in the comments on the frontside that you need to solder four extra cables but I didn't find any diagrams regarding this)