Mug-cake mix at home I would like to make my own mix for "mug cake" -- that is to say, prepackaged instant cake mix you dump into a mug, add liquid, and microwave. 


Is there anything special about mug cake that makes it cook in the microwave better? Will ordinary box cake scaled down work?
I'll want to make my own mix. Is there anything about boxed cake mix that's special beyond just sifting dry ingredients together, maybe throwing in some powdered milk so I don't have to use milk as my liquid? And can I do anything about requiring eggs -- for example, using some kind of powdered egg?
Are there any types of recipes for which this simply will not work? I kind of suspect Angel Food cake won't work in a mug :)
Not really.  If you mix up normal cake batter with the directions, and microwave it, it will cook.  It doesn't taste like a baked cake, but its certainly edible.
Actually, specifically, you add Angel Food cake to other cake mixes.  The reason is that angel food cake mix usually contains powdered egg whites.  The other option is indeed powdered egg, but angel food cake mix is more common in these recipes.
Mainly its about expectations.  Many things 'work' in the sense that they're edible, but they may not resemble the product you think they should.