If you put Robin Williams in as Zaphod, he would be a most frood dude, but it might be uncomfortably close to playing Mork, which I think he's worked long and hard to get away from in his subsequent career.

I agree about Michael Keaton. If he could handle Beetle Juice, he could handle Zaphod Beeblebrox.

Also, where could you put Rowan Atkinson? Is there any role he couldn't play in HHGTTG?