I can't really comment on Disk2fdi as I haven't tried it yet, but I'm sure there are some issues with it - there always is with new things.
The Blues Brothers disk on Amiga Action was a way of trying to get people to donate money to Comic Relief - they donate, they get the game. Someone somewhere must know the code I guess, it's the kinda thing someone would've written down on their disk label for future reference. Hopefully, it'll turn up some day.
The others that you mention sound like the Mortal Kombat II Competition Disk which was another Amiga Action one. It had a quiz on it where you answered the questions for a competition for something I forget - MK2 related I guess
You could also take the disk into Future Zone shops (in the UK) where you would get £10 off the purchase price of MK2.
Would be interesting if someone could "break the codes" tho
Finally, I have some disks which simply do not work in emulation at all, or at least have crashes/other problems during the game. Damn.
These are possibly emulation issues. Hopefully in time WinUAE etc. will run them