Alex,
This is an interesting question.
For the anecdote a good translation would be:
-He can eat it, and he would eat it, but who's going to give it to him?
This isn't a word-for-word translation, of course, but it fits the anecdote well.
If you want to consider Съесть-то съест, да кто ему даст? as a saying, apart from the anecdote, and you want an answer that sounds like a proverb in English, the translation is more difficult. You have to take into account the perfective aspect of the verb Съесть and the special emphasis of the particle -то. A more literal translation could be:
-He'll eat it up just to eat it, but who'll give it to him?
This translation implies that it is the nature of the elephant to eat that much, although no one is likely to give him that much.
Only a couple of equivalent English sayings came to mind, although I am sure there are more.
-If wishes were horses then beggars would ride.
An old English proverb.
And a more modern American saying:
-It's nice work if you can get it.
This saying refers to an easy way to earn a lot of money or a very desirable career. Everybody wants a job that will make you rich without much work, but very few people can find such a job.
He made a million dollars for a five minute appearance on television. It's nice work if you can get it.