In English “who” is used as a subject:
Who is eating?
“Whom” is used as an object:
The person whom I saw.
In Spanish is there any sort of a distinction like this, or is “quien” both for subjects and objects?
In English “who” is used as a subject:
Who is eating?
“Whom” is used as an object:
The person whom I saw.
In Spanish is there any sort of a distinction like this, or is “quien” both for subjects and objects?
No, there isn't. Interrogative words in Spanish don't retain any renmants of case distinction (unless you count cúyo, which I suppose we could call the genetive of quién).
In your two sentences, you'd get ¿Quién está comiendo? and La persona a quien vi.
Note, however, that Spanish greatly prefers using que in relative clauses, so while la persona a quien vi is fine, you will rarely hear it in most dialects, with la persona que vi being the preferred construction.
My intuitive way of thinking about this is yes, because the who compartment in my brain has quien in it, and the whom compartment has a quien in it. And I think that when an English speaker learns Spanish and gets the hang of saying a quien when it is needed, that person has an easier time knowing when one could use whom in English, and more importantly, when not to use whom. (For those who aren't aware -- most native English speakers never use whom at all. I used to think it was ignorance only that led to people not using whom. But at ELU, there's a strong anti-whom camp, which includes some of the top, most knowledgeable participants.)
Examples:
¿Quién está llamando a estas horas?
¿A quién le importa lo que yo diga?