An alternative to Unity's WaitForSeconds yield instruction that invokes a callback during each Update.

DoForSeconds.cs

/// <summary>
/// Yield on a DoForSeconds object within a coroutine and the supplied callback method
/// will be invoked during each Unity Update cycle.
/// 
/// Example:  yield return DoForSeconds (1, (elapsed, duration) => { Debug.Log (elapsed/duration); });
/// </summary>
public class DoForSeconds : CustomYieldInstruction
{
	public delegate void UpdateCallback (float elapsed, float duration);
	
	float elapsed;
	float duration;
	UpdateCallback callback;
	
	public DoForSeconds (float duration, UpdateCallback updateMethod)
	{
		this.duration = duration;
		this.callback = updateMethod;
	}
	
	public override bool keepWaiting {
		get {
			elapsed = Mathf.Clamp (elapsed + Time.deltaTime, 0, duration);
			callback (elapsed, duration);
			return elapsed < duration;
		}
	}
}