Eyeblinks, retopology, supratarsal fold- best practice?

Intertricity

I've tried thinking about all kinds of approaches- manually inserting more edges when doing the morph itself in the closed position, modeling in the eyelid fold so there's more geometry to pull down, modeling the eyelid half closed, etc.

What are the best practices when you need to do eyelid blinks with more complicated/realistic faces for a game engine?

dirigible

If you're concerned with realism on that scale, you're going to want the following effect:
The appearance of a fold of skin when the eye is open
The fold disappears when the eye is closed.

Usually, wrinkles and folds around the eye are handled with texture. If wrinkle-maps are an option, that's the route I'd go. Otherwise, you're forced to use geometry.

For geometry, I'd model the eyelid half-closed. You could rig up something in maya or max that allows you to add/remove geometry while toggling the blink blendshape on and off.