Acid inhibits gluten formation, so it's possible that incorporating an acidic ingredient such as buttermilk, vinegar, or lemon juice will tenderize the cake.
This will definitely force you to experiment and adjust your recipe, however, since the acid will also interact with other ingredients, most notably leavening.
More acid means more reaction with baking soda/powder, which might lead to a collapsed cake (if too much carbon dioxide is produced before the batter has time to set up in the oven).
It will also, obviously, have an effect on flavor. Given enough of the acidic ingredient, its own flavor might even become too prominent.
It should be noted, though (as Aaronut already mentioned) that cake batter just needs to be mixed until all the ingredients are combined. As long as you have no pockets of dry ingredients, your cake should bake perfectly well.