If you hide a Back Off at one battlefield, can you reveal it to stun a unit at another battlefield?

No, you cannot.

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."

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

This is also supported by RiftJudge FAQ #10524, which explicitly states: "No, you cannot use a hidden Back Off to target a unit at a different battlefield."