I myself enjoy enjoy the female singing voice, in general, more than the male, so I have always been searching very hard and haven't come up with much! But...
Celestial Horizon is a great example of a band with DT covers with a female vocalist. She has a lower range and personally I sometimes feel she lacks emotion, but it's still great to listen to. They have a downloadable "Tribute to Dream Theater" album featuring many parts of Metropolis Part 2 and a selection of other DT songs (with a few other covers).
Also, I know this girl(friend) who loves DT and she's an opera singer. I have encouraged her to record her own takes, and she has to some degree, and although she isn't proud of what she's done, I'll
link her anyway (her operatic style is very different, obviously). If she gets enough support she'll probably be more motivated to take it more seriously and put up entire songs!