French Meringue Buttercream

If you can pipe it well at room temp when you're decorating your cake, there's no reason it shouldn't keep well for at least a few hours at a comparable temperature. Cold buttercreams don't spread well, and cold cakes don't usually taste as good. I think it will hold as long as your room temp is cool enough - under 74F according to serious eats. I've worked with french buttercreams when my room temp was closer to 80F and it was definitely too loose.