I remember people reprogramming these back in the mid 90s when phones were relatively expensive, theft was rife, and reprogramming was really 'useful' to criminals.
Pre internet days I wonder how your average 'bloke in the pub' or 'friend of friend' managed to get the info/tools to do these tasks.
I suppose asking around in real life rather than the internets.
I think you need to look harder to find the information you're looking for.
But I can't see the point really. Buy a used phone from a random swap shop with cash if you want an 'anonymous' IMEI.
If you're really trying to do something dodgy, I think this is why most communities will let you do the leg work to figure it all out, rather than help you do dodgy things.
If you said what you wanted to do and it sounded reasonable, then help may follow.
Maybe worth looking at the new Linux phones, looking in the source code? At some point IMIE data must be passed, likely unencrypted, to masts.
A possible vector to spoof before transmission? Or spoof the value in memory, or some such?