She absolutely can appear in another game. Until we verify whether or not she's covered any wars, she can show up anywhere. Especially another Mercs campaign or a single player short set of her previous and then current mission.
+1 for Rachael. She deserved more.
Frank West needs to be playable DLC in RE Rev's Raid Mode and RE:ORC's Hero's Mode! Thank goodness he made it into uMvC3. He's covered wars ya know?