I've made my homework but I found no mention of this feat to be doable with a wordpress plugin.
However, you never know, my google-fu sucks, according to everyone who saw me in action, so I come here for a confirmation, please :)
In short : please, is there some way to create/update/hack a wordpress user rank so that he/she may ONLY act on the Tags of a post, adding tags, removing tags, and then saving the post ?
Not allowing a user to update anything else but the tags section.
I doubt that's possible, but I'd be glad to be mistaken !
Thank you very much if you've got an idea about it, or even, who knows, the magical-plugin-that-I-failed-to-notice able to do the work ;)
Good day to everyone,