It's such a well known issue/feature that it's become something of a meme in this forum.
The short answer is that when using s:link or s:button inside iterative components, you can't pass method parameters unless the list that you are iterating has been annotated with @DataModel.
The long answer is to do a search on @DataModel and s:link and read section 32.1.2. Limitations and Hints of the docs.
Shame on me ... I had not seen I was reading an old Seam documentation with no part about JBoss EL. As you said, everything is explained in the lastest one !
Thanks a lot for your help,