It is rare enough with American orchestras to appoint a principal guest. The reason for the post more often than not is to fill in something lacking in a music director. The last time the L.A. Phil had principal guests was three decades ago, when it brought in the especially versatile and tuned-in young conductors Michael Tilson Thomas and Simon Rattle to complement an old master, Carlo Maria Giulini. In the case of Mälkki, the natural first reaction may be that she’s a woman. While American orchestras have begun hiring female music directors, this is the biggest crack, thus far, in the increasingly fragile glass ceiling about to crash down on top-tier orchestras.