Thomas Middleton and Thomas Dekker's The Roaring Girl (ca. 1610) is a play that points out just how scary this malleability can be, and how wide the social implications of that malleability can spread. It is a fictionalization of real-life cross-dressing thief Mary Frith, who is Moll Cutpurse in the play--"Moll" being shorthand for prostitute and "Cutpurse" referring to the fact that her method of choice for her thievery involves cutting her victims' pursestrings with a large sword. In the beginning of the play, young lover Sebastian is telling his beloved Mary (who is also "a moll" though not the "mad Moll" of the play's title, and the link between the name Mary and prostitution suggests the need for the virgin/whore dichotomy to stabilize gender) that his father has canceled their betrothal because he is now dissatisfied with her 5000 pound dowry and thinks she is a social climber. In order to distract his father from the fact that he still loves Mary and is seeing her in secret, Sebastian says he is in love with Moll Cutpurse, which makes his father devise a plan to break up their (nonexistent) union, thereby enabling Sebastian to see Mary undetected. Though Moll does not desire to enter into the institution of marriage herself because she thinks it "is but a chopping and changing, where a maiden loses one head and has a worse one in th'place," she supports and participate in the plan to rejoin the lovers, enabling them to marry by convincing Sir Alexander (Sebastian's father) that she has run off with Sebastian, causing Alexander to say that he'd like anyone but Moll to marry his son, which allows Mary to enter into her union with Sebastian as originally planned. I think that quote is so great because, in one pithy statement, she attacks the commercialization of women, the importance of virginity to a woman's worth, and the church's role in patriarchy.
The thing that seems to differentiate this play from a number of cross-dressing comedies of the period like As You Like It or Twelfth Night (or even Book III of Spenser's Faerie Queene, though that's not a comedy) is that Moll cross-dresses because she knows men's clothes endow those who wear them with social power. This is in keeping with the way Butler says gender formulations should be questioned. The play's great number of fashion metaphors, the first of which appears in Middleton's epistle to the audience, shows this. She doesn't do it in service of her own desire to enter into traditional marriage, as Rosalind, Viola, and Britomart do. She wears pants, yes, but she doesn't try to pass as a man. By wearing pants but also doing nothing to physically disguise her womanhood, she confuses the people around her by being two things at once, or "not all man and not all woman" as Sir Alexander says. This destabilizes the stringent ways such people think about gender. At the play's end, Sir Alexander respects Moll and admits that public opinion about people can be wrong, so Moll is able to affect real change.
I think The Roaring Girl is a better protofeminist play than As You Like It or Twelfth Night for another reason as well: the way it exposes constructions not only of gender, but also of class. In embodying traditional pastoral escapism in their cross-dressing adventures, both Rosalind and Viola play at being of a different class. By the time their plays end, though, they are both married to men with high social stations. Everything is stable and playtime is over. These two plays could be said to explore class through Celia or Maria, but both those women are limited by their class positions. They don't get to move around to different social positions, they just tag along while Rosalind and Viola slum it. Conversely, the three merchant ladies in The Roaring Girl have their own plots that work to question the stability of class and the way it overlaps with the stability of gender, as in the fabricated affair between Mistress Gallipot and Laxton.