Can I use a hidden Back Off on another battlefield to stun something when I move in?

No, you cannot.

According to Rule 811.1.d.2, if a hidden spell chooses any targets, those targets must be chosen from among options at the same battlefield where the card was hidden, unless the card explicitly contains a targeting restriction that makes this impossible.

RiftJudge FAQ #10586 and RiftJudge FAQ #10524 explicitly confirm this: you cannot use a hidden Back Off to target a unit at a different battlefield.

If you attempt to use it as a reaction to moving into a battlefield, the Back Off must have been hidden at that specific battlefield to be a legal play. Because Back Off is a spell that targets a unit and does not contain a targeting restriction forcing it to target other battlefields, it is restricted to the battlefield where it is currently hidden.