You DO know that disabling the IMMO takes 30 seconds over the OBD port with the right tool right? 

Just a few thoughts on this, 

-Yes, it is possible to read the me7 eeprom via obd and disable the immobilizer with some tools regardless of if they are matched or a valid key is used.  This I have done and know sure.

-If you're really worried about someone disabling the immobilizer over obd why not just install a K wire kill switch somewhere.  It would slow someone down, and they probably wouldn't hang around to check wiring.  If they really want your car they'll come with a spare ecm with the immobilizer off or a flatbed but it's a easy inexpensive way to make it harder. 

-As for actually adding the immobilizer to an s4, when matching the ecm and cluster I believe there are more than just the vin and skc.  To use vag commander nomenclature there are component security and mac bytes .  I started doing some experiments on this but lost interest. I will take a look if you guys would like, or maybe you already have this figured out.  From what I remember it didn't appear be as simple as putting the same values in both the ecm and cluster but I'm not sure.  To further complicate things I'm fairly certain you can only replace 1 component (ecm/cluster/key) at a time or else you need to replace all with new previously unadapted parts.  I know vag commander has functions to reset the ecm or cluster to virgin state but I haven't personaly tested these functions.  Again, I have some cluster bin files showing various states of being adapted so I could try to find various values in the bin file but I haven't had time to really disect them.  If it will help I'll try to find them and see what I can get out of them.  All of my research files are from golf/jetta cars with vdo clusters just to warn you.  Who is the manufacturer of the rs4 immo clusters?