Don't know anything about Bob Hamlin's status, but he's also the only unknown for Alice in Wonderland, too, where the Mellomen played the cards that were painting the roses red. Kathryn Beaumont (Alice) is still alive and turning 80 this year, but she's the last confirmed survivor; Hamlin's a question mark, and everyone else is gone.
On the topic of Disney movies, Fantasia is an interesting one. None of the six vocal performers from the original 1940 version are still alive (the last one was Julietta Novis, the 'Ave Maria' singer, who died in 1994). But the master tape of Deems Taylor's original voiceover narration has deterioriated so badly that they can't use it anymore, so the narration has been re-recorded by three other actors since then, most recently by Corey Burton in 2010 - and all three of those actors are still alive. So I don't know if you want to count that one or not.
Disney's Swiss Family Robinson (1960) only has one surviving cast member: Tommy Kirk, who turns 77 this year.
Outside Disney, I also looked up the three Topper movies, which I thought might be good candidates: Topper (1937), Topper Takes a Trip (1938), and Topper Returns (1941).
Topper Returns yielded a hit - in fact, the last two cast members of that movie (Patsy Kelly and William H. O'Brien) both died way back in 1981.
The other two movies each have one unknown gumming up the works, with everyone else confirmed deceased. In the original Topper the mystery actress is Donna Dax, who played a minor role. Her name also pops up in several other famous movies from the late 30's and 40's, most notably Citizen Kane, so she's someone who could end up haunting this thread if no one can find any information on her. Topper Takes a Trip has the more familiar problem of a total unknown named Jimmy Colman, who played a waiter in his only film role.