Is it acceptable to use a negation after "unless"? I know it's far from standard.
You don't love people. But you will succeed unless you don't like people.
There isn't any reason not to use a negative after unless. I can give examples unless you'd rather not hear them. :) "We could go out for pizza, unless you don't want pizza?" "You might try using cilantro, unless you don't have any. Then you might try parsley." As a matter of fact, I don't see anything wrong with the example in the OP.
I'm afraid it won't be a preferred version. It's not that natural as compared to...
You'll not succeed unless you love people
Note: I've not factually examined this sentence!
Most grammar books says avoiding two negatives to make things positive.
Prefer It's common over It's not uncommon.
Unless you play badly, you will win the game
Is the same as
If you play well, you will win the game.
Unless - except under the circumstances that
Ex: Exceptional talent does not always win its reward unless favored by exceptional circumstances.
From The American Heritage® Dictionary of the English Language
Unless implies a negative impression. It means "if not the case that..."
If the clause after "unless" is negative, the negative of the clause and the implied negative of "unless" makes the total positive. It's the case of two negatives makes positive in a sentence. And in standard English we generally avoid these kind of construction. Though they are not uncommon, especially in some circumstances it does make more sense.