What could be used as a replacer for the egg in Yorkshire puddings, having tried before nothing quite seems to work properly :(
In a pop over-type batter, the eggs provide a few functions:
So, my suggestion would be to up the liquid in the recipe by a few tablespoons, add a little bit more fat, some powdered lecithin (or other emulsifying agents -- you can get vegan lecithin made from mustard), and some soy protein powder.
You might have to play with the combination of these ingredients, but by replicating and replacing the items the egg provides, you'll be much better suited to creating the popover.
Although, you're not making yorkshire pudding FYI -- you're making a popover. Yorkshire pudding is a popover created by using the rendered fat from a roast. A popover is the same type of bread, but made without using animal fat.