Is it possible to build across all selected buildings simultaneously?

Solution 1:

Probably not exactly what you want, but you can also hold down the hotkey, and it will rapidly queue up build orders. It can be pretty handy for things like morphing a lot of banelings.

Solution 2:

StarCraft II has a new model where you tell it exactly how many things you want. If you want 5 marines, you press "Marine" 5 times. If you have 10 corrupters selected, and you want 4 brood lords, press Brood Lord 4 times. It is a little annoying at first, but is vastly superior. Imagine you've got 3 larvae selected. You can press "drone", "zergling", "overlord" and it will build one of each. Very handy.