IMO it's not rude to ask politely for this, or for almost anything else. Like a seat swap, you're asking for a favor, so you should accept a refusal gracefully, not act entitled, and especially not put your laptop in that space before asking.

If I were in the row behind you, I'd probably say no, but only because my soft-sided briefcase is usually jammed so full that it takes up the whole space, not because it's an unreasonable request.