No. And keep in mind that if he had, Ruby would have admitted it. Why not? He was taking the blame for shooting Oswald, so why lie about working for Nixon? Plus, Ruby was a blowhard. He loved to talk. He loved to brandish his connections. He would surely have admitted it. But, in the autobiography that he wrote with the help of that television scriptwriter, he said nothing about working for Nixon.
This is from Jefferson Morley of JFK Facts:
This letter is fake. They didn't even have 5 digit zip codes in 1947, which this letter has.
And, they didn't even have zip codes in 1947.
So, do you know what this tells me? It tells me that the whole "Ruby worked for Nixon" claim is just a baiting, battering, and buttering up of the buffs thing. It's just more of the false and phony lure about Jack Ruby, the invented biography, that he was gangster, a pimp, a guy who threw people down the stairs, a gun runner, and an operative for Richard Nixon. In reality, he was just a nightclub operator in Dallas who got a thrill out of being friends with the Dallas Police. Most of what you hear about Jack Ruby is patently false, including that he shot Oswald.