Guinan is a member of a long lived species and we know from TNG she had been to Earth as early as the 19th century so squeezing her into the 22nd century wouldn't have been difficult.Really wow Guinan wasn't she in TNG and Q how are they able to use characters that weren't even around yet I never quite got that
And I'll add that Q is from the "Q-continuum" -- a race of beings that evolved to become immortal, powerful -- basically like Gods. Time and space are irrelevant to him and his kind.
So yeah, it would have been perfectly fine for them to have been in an episode or two of Enterprise.
Personally, I'm very disappointed about not getting the MU goodness. *sigh*