Has anyone swopped out an ECU with built in Immobilizer?
If I get a second ECU will I simply be able to "pair" my current keys with it?
Next question, will the keys then work with both ECU's?

I have no idea about your question, but was wondering: Do these keys have a chip in them? I don't think so but obviously I don't know for sure.....

I think they do have chips.

And I'm not talking Snyder's of Hanover, either.

Yes, the key has a transponder in it. It communicates a unique ID with the ECU's ID when you insert the key. If matched, you can start the engine. For some reasons, US models don't have it (Connie may have it).

KawasakiWorld is a forum with a large contingent of europee-ing riders. There's at least one thread directly involving ECU "swopping" with imobilizers.

At one point, some limeys rode into France on their 12Rs. Some French parking lots have a damping field generator... supposed to combat some kind of theft nonsense.

The net result was these guys bikes shut down in close proximity to the lot because the damping field interrupted comms between the key and ECU.

You need a paired key and ECU. The immobilized key shouldn't work with your non-immobilized ECU. Immobilizer isn't a perfect solution for theft, though. If thieves physically move your ZX away and use so called "immobilizer cutter", they could pair a new key with your bike's ECU. Maybe this is why we don't see too many of immobilized motorcycles here in US. There are many strong dudes. THe best way to protect your bike is to use a good lock and keep it in a garage at night.