I had to phone a friend on this one, as literature/mythology of eunuchs is not my strongest suit (being more a just-the-facts-ma'am kinda historian), but my friend does specialize more in this area, and he says that the primary representations of eunuchs in plays of this period is comical, they didn't really get a serious enough treatment to get into zones of good and evil. A few examples:

From him: French fabliaux will use castration for humor:

The fabliau "The Fisherman from Pont-sur-Seine" involves a man who tries to make the point that his wife only loves him for sex. She protests, but he comically proves his point by pretending to have had his penis severed, at which she immediately renounces all interest in him. The ruse requires the use of a dead priest's organ, discovered by chance in the river: "At that he took his fishing knife, / Clipped the priest's penis at the root, / Washed and dried it in the boat, / Put it in his lap, and fished..."

The fabliau "Aloul" involves a priest who has sex with a married woman and hears menacing shouts such as: "You worthless people! What the hell / Are you doing? Help me! Hurry up! / Everybody come and help / Castrate this priest!" and "Don't wait-- / Go get a razor. We'll castrate / This priest and then go back to bed."

I've also got an example for you in English, which is Marlowe's A Game at Chess, and has castration used as a metaphor, with not really any humor. I'm not sure if I'd say this is really about eunuchs, as it's a political allegory, and the act of castration is in this case more meaningful than the status of "eunuch."

I don't say they'd be that common in plays, from the few my friend and I can scrape up for you. Outside of plays, I know of one instance of a eunuch character in an early (17th c.) Italian opera, and it's comical. I've got a few more examples for castration in plays off the top of my head for the 18th century as well.