I actually think that Sonny might have been able to pull that off. Not, of course, in the sense of wanting to betray his family since that wouldn't likely be believed (although not outside the realm of possibility) but I think he could have presented it as him wanting to get a little side action under the table without Don Corleone knowing about it. "Pop doesn't like the drug racket, but I think it's gonna be the coming thing in the future. So I'd like to, ah, you know... wet my beak a little on my own."

Ultimately though the issue is the minute that Vito refused Sollozzo, it was a declaration of war. So in the end no matter who went to try and get information about Tattaglia, it would probably have been suspected immediately as a Corleone plot because right after the refusal it was determined that Vito had to be killed. Seeing as Vito was not stupid, any Corleone person, even some lowly button man, asking questions would be suspect and probably killed off just in case they were a spy.