This thread has become about more than simply unselfing the lv2diag.self file, altering the authid, and re-selfing it.
In the process, we've discovered problems with both the makeself and unself tools. slynk has been working on fixing these issue, and much of the thread is dedicated to that effort.
As for the original downgrading self, there are modified tools (sceverify) that can take the original self, hex-edited to change the authid, and re-sign it (no decryption/encryption required, since the authid is not part of the encrypted portion). Of course, then the problem came up that Sony fuxxored consoles that had 3.55 installed to not downgrade properly, anyway. Thanks Sony!
Still, we want to be able to create and use NEW lv2diag.self apps to help with diagnosing problems, exploring the "raw" console and perhaps live patching the kernel for hacking.
Assuming they simply revoked the original by hash, it should be a simple matter to decrypt (is it encrypted?), modify a character string somewhere in the binary, and re-encrypt and re-sign it with the proper keys.
Then it's a simple test to try and downgrade a 3.55 upgraded PS3.
I really need to get these tools together, but I can only mash my way through Linux (Mostly a Windows dev for almost 20 years) at the moment, perhaps when evilsperm sees fit to make another update to his ubuntu virtualbox image, I can finally get cracking.
At any rate, it seems like a simple enough thing...