Can I use hook_uc_add_to_cart to ask for a yes/no confirmation before adding to cart?
I was able to solve this issue using the first answer on this stackoverflow question : https://stackoverflow.com/questions/10490253/how-to-save-a-session-variable-in-drupal-7

In general: Drupal's static $_SESSION array variable allows to store key=>value pairs between different node pages, in my case the same page.

My code snippet inside uc_product_minmax_uc_add_to_cart hook :

    if (!empty($disclaimer)) {

        $confirmed = lists_session("s_key");

        if(! $confirmed) {

            $message[] = t($disclaimer, array('@qty' =&gt; $minmax-&gt;pmin_multiple, '!item' =&gt; $node-&gt;title));
            $result[] = array(
                'success' =&gt; FALSE,
                'message' =&gt; theme_item_list(array('items' =&gt; $message, 'title' =&gt; t('Read this note from medical department before confirming your reservation :'), 
                'type' =&gt; 'ul', 'attributes' =&gt; array())),
                'silent'  =&gt; FALSE,
            );
            lists_session("s_key", TRUE);
        }
    }