Stalemate timer forcing mutual retreat: Mutual retreat is essentially defender victory, isn't it? Although with some possibility of surviving units on both sides. It might make defensive run-around a winning strategy.
Landless battles don't require you to have buildings (which couldn't run), so I'd expect evasion/stalemate-exploit to be possible. But the AI wouldn't exploit that(?), and humans competing in good sport shouldn't, so maybe that's good enough.
However, a terrain-based stalemate resolution, complex or simple, would more fairly judge a coast battle where the defender holds only the sea or air (with no ability to attack ground) and the attacker holds all land.
Maybe it is that simple. Each battlefield has a main terrain (land, water), and if one side has units on this terrain and the other does not, the side with these units is judged the winner of a stalemate, forcing the other to retreat. If neither side has this advantage, they both retreat, and the area of the battle does not change owner (land tiles: defender keeps it).
