The fact that a relative clause with a plural verb can modify a coordinate structure consisting of singular NPs is evidence that the coordinate structure itself (the higher NP comprising the coordination of the two NPs "man" and "woman") can take an adjunct. If it can take a right adjunct, why couldn't it take a left (attributive) adjunct?
That said, I think the following are awkward at best:
(i) I saw a man and woman who was black.
(ii) I saw a man and black woman.
And the color issue arises for me there. Why go out of one's way to state the skin color of one without stating the skin color of the other? 99bottles, however, wants to state the skin color of both. I could make up contexts all day long in which that would make sense.
I wonder if (i) and (ii) are grammatical. Do they absolutely need to be "I saw a man and a woman who was black" and "I saw a man and a black woman" instead? I tend to think that, although (i) and (ii) are extremely awkward, they are grammatical. How about you? Feel free to replace it with the taco-and-enchilada example.