Cobweb (with potion of weaving) generating on happy ghast

Until we have a new block like the Floatato, it could be useful to have a way of placing a block in the air while on a Happy Ghast, since they are designed to build in the sky. But just right-clicking a flying mob to place a block would be too easy.
So I tried to spawn mobs on it, by throwing chicken eggs on myself, or by drinking potions of Infestation and Poison.
Then I threw a potion of Weaving at them, and killed them on the surface of the Happy Ghast. But it doesn't count as a real surface, so no cobwebs in the air...

But I think it could be useful for mobs affected by weaving to generate cobwebs ON the Happy Ghast upon their death, as an alternative to build up from the ground to start building in the sky. 
I think that could be a balanced alternative, since it could save you some time, but only if you have prepared for it before.