Solved ! I just had a misplaced dot preventing the whole .js to work. Once deleted, the client side method works like a charm.
My mistake was revealed by OSX less.app which, in such cases, returns an alert.
However, I'm still unable to get the WP-Less plugin working...
Following the installation notes, here is what I declare in my header :
<link rel="stylesheet" type="text/css" media="all"
href="<?php wp_enqueue_style('template_name', get_bloginfo('template_directory').'/style.less', array('blueprint'), '', 'screen, projection'); ?>
When I take a look to the generated source, I get