tldr: “do not make a rape victim the butt of the joke, QED”
I think that applies quite well to this topic; dark humor ceases to be funny when it becomes an actual attack against another person.
But what if someone told a joke that made fun of rape victims that also caught people by surprise and made people laugh? Trying to create some arbitrary lines about what kind of comedy is permissible and what kind is forbidden just makes the forbidden stuff into more of a sacred cow. And humor is all about slaughtering sacred cows.
Rape is about power. Power conceded by the victims and forcefully taken by the attackers.
A rape joke could make fun of how, useless, sad, an worthless a person would have to be to force themself on others.
How the act itself is nothing but a pathetic act of desperation.
Use perspective, artful storytelling, crafty dialog/writing to take the power from the attackers. Face the fear behind their actions and reduce it to reality; that rape is a sad, terrible, pathetic thing to do to somebody.
Take away the power from fear and gove it back to the victims in the form of empathy and support.
The 'sacred cows' as you say aren't just topics with shock value. They're topics that people are afraid of. Whether because they're taboo, morrally wrong, or legitimately terrifying.
Some may say that the victims suffer more due to living dealing with the shame and lying to loved ones to keep their past a secret.
In the right hands a truly talented comedian could make a great rape joke that is actually empowering to rape victims. One that makes the topic less taboo and helps victims feel less shame in a past that they had no power to shape.
Tasteless humor is noyhing but an ad-hominem dressed up with a fancy delivery. Good comedians don't attack people, they shine light on the ridiculousness of life and in doing so bring people together.
If that were true, South Park would not be successful.
Reduced to childhood terms, they're the kids on a playground playing doctor. Yet, as adults we treat people who play doctor with equal or higher regard than those who actually are doctors.
It's pretty idiotic but we all tend to attribute image as a viable replacement for substance.
South Park just points out the ridiculousness of it all by poking fun at those who take their image too seriously or make a mockery out of their status as a public figure.