For me it depends on the woman.
I don't like female death metal vocals, or screechy vocals, not because tehy are female but because they are death metal and/or screechy. But I could listen to Ann Wilson or Johnette Napolitano all day long. I would think that of any prog metal band, DT is most set up for having a woman singer, and I think an Ann Wilson (or Gretchen Wilson, though different genre) could really do some of the more melodic choruses justice.