Can I cast a Hidden Back off into a different battlefield?

No, you cannot use a hidden Back Off to target a unit at a different battlefield.

According to Rule 811.1.d.2: "If a hidden spell or a play effect of a hidden permanent chooses any targets, those targets must be chosen from among options at that battlefield, unless the ability explicitly restricts targeting in a way that makes this impossible."

Because Back Off is a spell that targets a unit, and it does not contain a targeting restriction that forces it to target another battlefield, you are required to choose a target at the same battlefield where the card was hidden.