Hi there all I know is that a lot of biscuits are made with yeast powder but I use self raising flour and take out ounce of it and replace it with cornflour to make the biscuit crispy. Hope this helps.
Egg is often substituted by some baking soda for those who have an allergy to egg, the baking soda replicates the basic qualities of egg white. By basic, meaning its pH. Substituting baking soda with egg doesn't work because the volume and the moisture don't account for the reason you're adding baking soda.
Egg does add its own structuring and lifting capabilities varying with the recipe. So it's really more important why you're trying to avoid leavening agents. If it's about sodium, there are sodium free baking powder such as Hain Featherweight. This is a single acting baking powder that substitutes quite well for regular baking powder.
Give popovers a try with a standard muffin pan. And if you like it, get some of these cast iron pans to get serious about it.
Get two. A standard batch of popover batter is based on 2 eggs. make a batch and a half and you'll fill two of these pans just right. Preheat them in the oven at least 20 minutes so they're good and hot. Cook's Illustrated disagrees with me and likes to use glass ones from room temperature in a cold oven. Just for full disclosure.
You can use a standard cupcake/muffin tin. The problem in my experience is that they don't have the thermal mass to do the job as well as these other pans and they release much more poorly. Less "pop" as well.
I'm not particularily fond of baking powder/soda, so I try and avoid it when I can.
When I make scones, I use a weird technique. If you're new to baking, you might not know about puff pastry, but there are a zillion ways to make it. One way is the "Blitz" or lightning method, where you cube your butter and mix it into the flour, add your water, and make a right mess. Then you roll it out, fold it, and repeat a few times. It rises quite well--not as good as classic puff pastry, but still quite well. What happens with puff pastry, is you have a thin layer of dough and a thin layer of fat (usually butter), paper thin layers,actually. When you bake, the water in the dough turns to steam, but the fat layers prevent/slow it down from escaping, giving you a decent rise. As it continues to bake, the steam eventually escapes and the fat melts into the dough.
So I use this technique with my scones. If I'm in an extravagant mood, I'll whip some cream, and fold this into the dough--adding another shot of air, which rises when the heat hits it.
Being a Canuck, I confess I don't know much about "biscuits", but it would seem that the doughs of biscuits and scones would be somewhat similar.