Hi Michel.
Looks like Lilly of the Valley grows best between zones 4 through 8. Belgium is 6 so you should be ok. I would try the drip method using an inert medium.

No, they ought to do quite well, we had a dripping faucet and ours multiplied extensively

I was wondering the same thing. Lily of the Valley perfume is very expensive and I was thinking about starting a business.