From Memory Alpha:
Guinan was slated to appear in an episode planned for the unproduced fifth season of Star Trek: Enterprise entitled "The Treatment". Under the assumed name "Claranna", she would have been acting personal assistant of the head of the United Earth Space Probe Agency. More of her background would have been revealed, including what she was doing there since the time of Mark Twain.
Interesting, though I'd be curious to know what MA's source is for this info.
And needless to say, Guinan wouldn't need to be on Earth to play part in an ENT