[eru]

> I always thought of plain old hitboxes as being unnecessarily cruel - the player can't see them [...]

Err on the side of the player. Ie give the player sprite a little smaller hitbox than would be warranted by pixel perfect collision detection, and the enemy sprites a little bigger one. That will feel very fair, and you can ramp up the difficulty in other ways.

Following the same principle, Canabalt famously allows you to jump a little bit after you already left the rooftop.