Is the premarital unchastity case of Deut. 22:13 an example of the justice system assuming guilt until a defendant proves her innocence?
No. This is a section of the case laws that is often misunderstood, because it is actually intertwining the circumstances of two different prosecutions (legal cases). These cases are not describing a woman who is (as one well-known commentator claims) "presumed guilty until she proves her innocence".
Here are the relevant verses:
13 If any man takes a wife, and goes in to her, hates her, 14 accuses her of shameful things, gives her a bad name, and says, “I took this woman, and when I came near to her, I didn’t find in her the tokens of virginity;” 15 then the young lady’s father and mother shall take and bring the tokens of the young lady’s virginity to the elders of the city in the gate. 16 The young lady’s father shall tell the elders, “I gave my daughter to this man as his wife, and he hates her. 17 Behold, he has accused her of shameful things, saying, ‘I didn’t find in your daughter the tokens of virginity;’ and yet these are the tokens of my daughter’s virginity.” They shall spread the cloth before the elders of the city. 18 The elders of that city shall take the man and chastise him. 19 They shall fine him one hundred shekels of silver, and give them to the father of the young lady, because he has given a bad name to a virgin of Israel. She shall be his wife. He may not put her away all his days. 20 But if this thing is true, that the tokens of virginity were not found in the young lady, 21 then they shall bring out the young lady to the door of her father’s house, and the men of her city shall stone her to death with stones, because she has done folly in Israel, to play the prostitute in her father’s house. So you shall remove the evil from among you. Deuteronomy 22:13-21WEB
Disentangling the cases
In the first case (the father accusing the husband of slander), the "signs of virginity" will guarantee that the father wins, given that the husband has no real (eyewitness) evidence, and is just publicly slandering the daughter. If the father chose not to keep such "signs," then it would only mean that he couldn't successfully prosecute the husband for slander.
The reason that the "signs of virginity" (a cloth with blood, perhaps?) is mentioned in the second case is that it would act as clear exculpatory evidence on behalf of the girl (in a circumstance where the husband used false witnesses).
In Deut 22:20, the "signs of virginity" are merely functioning as a rebuttal to the husband's evidence. He must already have provided at least two eyewitnesses, because two witnesses are always required:
15 One witness shall not rise up against a man for any iniquity, or for any sin that he sins. At the mouth of two witnesses, or at the mouth of three witnesses, shall a matter be established. Deuteronomy 19:15WEB
Deuteronomy 19:15 states that legal "truth" had to be established by 2 or 3 witnesses (confirmed by Jesus in John 8:17). This always puts the burden of proof on the accuser, never on the accused.
These witnesses provided by the husband would presumably claim to be witnesses to, or participants in, premarital fornication by the wife. The bloody cloth (assuming that's what it was) was considered a credible and sufficient defense by the girl and her father against the charge of lying about her virginity. Notice the two parts of verse 20:
20 But if this thing be true, and the tokens of virginity be not found for the damsel:Deuteronomy 22:20KJV
The phrase "the thing is true" shows that the husband's charge was established by eyewitnesses as "true" (in the sense of legal testimony which has not been rebutted). The lack of signs of virginity would not qualify as any sort of "witness" -- it is merely an absence of evidence. Therefore, we can infer that two or more actual eyewitnesses to the girl's guilt were brought by the husband. The phrase "evidences of virginity are not found" shows that there was no exculpatory evidence to defend against the eyewitness testimony.
This is not a case of indirect(circumstantial) evidence being used to prove guilt. It is a case of indirect evidence being allowed and presumed credible as exculpatory evidence showing the girl's innocence.
- Walter Kaiser, Toward Old Testament Ethics, p. 229