Meringue can be made with Splenda or any other artificial sweetener - I think in the interest of eliminating any grainy texture it would make sense to use liquid or concentrated drops as opposed to the granular versions, which often are mixed with maltodextrin anyhow - so you want to avoid that.
The icings I've made since D are all made with sweetened/flavored cream cheese. I haven't tried to make artificial powders act like confectioners sugar, but it may be possible. They're expensive to begin with, and I haven't had the discretionary income to invest in what could possibly be a pretty large failed experiment. But that's just me - others will chime in here I think.
I also think it would be pretty difficult to substitute artificial sweetener for powdered sugar in frosting. If a frosting recipe calls for granulated sugar, if you use one of the sweeteners that are used cup-for-cup, those contain fillers and the ones I have seen are pretty high carb. Here is a link to the cream cheese frosting recipe I use. It calls for 8 pkts. Splenda, which is fine for me, but you can sweeten it to your taste. I often substitute chocolate, strawberry or maple sugar free syrup for the heavy cream, and sometimes add coconut and/or chopped nuts, so it is a very versatile recipe, if you want to give it a try. But pretty much any low carb cream cheese frosting recipe is good.
I use a 50/50 mix of cream cheese and heavy cream, sweetened with Sachrin and NutraSweet. You can use Eryhthritol in its powder form as you would powder sugar. It does have an alcoholly feel about it though in your mouth.
I've read in some low carb forums that they use powdered erythitol in frostings. I think you can just put regular erythitol in a blender. Most of the recipes I have seen are bulked up with Cream cheese or Heavy Whipping Cream, which would need to be refrigerated once you make it.