Just found a quick way to have tags always right aligned, taking advantage of font-lock and display property.

(add-to-list 'font-lock-extra-managed-props 'display)
(font-lock-add-keywords 'org-mode
  `(("^.*?\\( \\)\\(:[[:alnum:]_@#%:]+:\\)$"
     (1 `(face nil
          display (space :align-to (- right ,(length (match-string 2)) 3)))
        prepend))) t)