Can you dig up some sharii rulings on witnessing in this particular context, because that rule is not hard and fast and applied to all sorts of testimony in real world Islam (present or past)? For example, in cases involving divorce and adoption / fostering, the testimony of one woman is equal to the testimony of one man. Before ever using something like this, I'd like a source to back it up first. The only thing I've ever read about witnesses to fornication (zina, not rape) is that it should be "four upstanding males," but there was no indepth discussion of what is meant by upstanding, or if one of the witnesses is a woman and three are men. I think the implication there was that when fornication is taking place in a location where multiple people are witnessing it, the witnesses most likely are not very upstanding people.
It might take a while to dig stuff up - actually I hadn't realised how many variations of rules of evidence there were within the various bits of law and from school to school.
Here is a bit taken from some anti-islam site - but I assume it's reliable:
The evidence required in a case of whoredom is that of four men, as has been ruled in the Qur'an (Surah xxiv 
; and the testimony of a woman in such a case is not admitted because az-Zuhri says,
"in the time of the Prophet and his two immediate successors, it was an invariable rule to exclude the evidence of women in all cases inducing punishment or retaliation," and also because the testimony of women involves a degree of doubt, as it is merely a substitute for evidence, being accepted only where the testimony of men cannot be had; and therefore it is not admitted in any matter liable to drop from the existence of a doubt. The evidence required in other criminal cases is that of two men, according to the text of the Qur'an; and the testimony of women is not admitted, on the strength of the tradition of az-Zuhri above quoted. In all other cases the evidence required is that of two men, or of one man and two women, whether the case relate to property of to other rights, such as marriage, divorce, agency, executorship, or the like.
Ash-Shafi'i has said that the evidence of one man and two women cannot be admitted, excepting in cases that relate to property, or its dependencies such as hire, hail, and so forth; because the evidence of women is originally inadmissable on account of their defect of understanding their want of memory and their incapacity of governing, whence it is that their evidence is not admitted in criminal cases. "The evidence of women is valid with respect to such things as it is not fitting for man to behold." Ash-Shafi'i holds the evidence of four women to be a necessary condition in such cases. 
Topic: Is it getting better with women's rights?