The biggest problem with retro-fitting cruise to a Laguna is likely to be the fact that most of the controls are in the steering wheel. To do it properly, you'd need to remove the wheel (the air bag should make it more fun) and replace it with one that has the switches. If you're clever enough and can find the wires that go to the wheel, you could build your own set of switches - each one just applies a specific resistance across the wires.
The master cruise switch should be easy enough to fit - on the phase 1 at least, the panel just pops out. I've yet to work out how to remove it on my phase 2 - I can't work out whether the silly bit of walnut trim has to come off first or not, but every method I've tried has failed to make the panel budge.
If someone with a cruise enabled phase 1 could meter out the cruise switch (and tell us which pins it connects) then maybe it'd be worth just trying that to see if you can get the green (cruise) or yellow (limiter) light to appear on the dash. If you can, then it may warrant further investigation. If you can't, then it's probably one of those things that has to be enabled by the dealer, and you might as well forget it.
In addition to the usual switches (brake, clutch etc.) the cruise-equipped Lag 2 has a two stage accelerator pedal - you can feel the extra resistance at around half way travel. This is part of the limiter functionality, and if you go past the resistance point, you override the limiter. You'd probably need to fit this type of pedal to get things working properly too.
Whichever way you look at it, it's certainly not going to be plug and play