Originally Posted by
MojaveFlyer
But you've got to clear the aisle, so hovering anywhere is going to be someone else's seat

So my approach is to stand at the seat I would like, with my bag in hand and not overhead or under seat. I hope that conveys that I am looking for a swap, but not being presumptive. And always, always, always, give up the ask promptly and gracefully, with a smile, if it is declined.
Standing AT the seat, okay. Occupying the seat, bag in hand or not, would trigger my "a-hole" complex. Being honest, pleasant, and transparent about the proposed swap will generally preclude that complex.