I'm pretty sure Meiling's mature and experienced enough not to need one of those. 4000 Years is way more than enough, and even if that's just a boast several centuries will just as easily suffice.
actually, she is a dragon. this is 2nd version of dragon. If she transform to full ver. of dragon, she's job might be over. Even Remi won't let a dragon do a gatekeeper job. and no one in Gensoukyo can force her do her full power...