Hmmmm interesting question. My guess is that it would be possible, as this is basically the process a dealer would go through to replace it. You'd need the keys, the ECU and all the lock barrels, that's ignition, doors, bootlid and glovebox. But in principle, yes I think it'd work.
Is the alarm and immobiliser built into the engine ECU or is it a separate unit - anyone?
Cheers MikeB. After speaking with Lexus and a couple of autoelectricians. I decided it was not worth the risk for a number of reasons.
1. The ECU's can differ year to year, country to country, man or auto trans. So the possibility that the ECU would be the exact match for my model and year of IS200 was slim.
2. The door locks are apparently a pain to change.
3. I originally thought it would not be that cost effective
Lexus service gave me the number of a company in Sydney which has the equipment to deprogram and reprogam and ECU and get replacement keys. I went for this option.
Cost? F'ing hell! It was not cheap. Price in Aus dollars; $880 to reprogram the ECU. $300 for the genuine 3 button lexus key. $60 to lasercut the key. 1 hours labour.
I got two keys so all up it was over $1800! I'm just repeatedly telling myself how more expensive and inconvenient it would have been if I lost my one and only valet key. I'm also spending "quality" time pressing the lock, unlock and boot buttons.
Apparently (I was told after the purchase) that you can buy the standard lexus key with no buttons for $47.