This is the site I go to for kitten glop. http://www.thunderkatz.org/glop.html
It can be used for all the reasons you mentioned. If your pregnant queen is needing more nutrients, or after she delivers. Kittens can use it if they are losing weight or sick. It is supposed to give kittens and mothers all the nutrients and calories they need, and it is easily digestible.
It wouldn't be something I would use on a regular basis, as the kittens may not want to go back or begin dry food.