Can I find unique dwellers in the outlands using the radio station?
I can confirm that you may attract unique dwellers on the radio as I got Butch just yesterday.
I have found between 20 and 30 Dwellers via Radio Station signals, and none of them have been unique characters. From what I have read on other sites, it appears unique dwellers can only be obtained via lunchboxes. If I see or find out otherwise, I will change this answer.
As an interesting aside: "unique" characters are not actually unique. Somewhat amusingly, you can find multiples of the same unique character in lunchboxes.