Can the outside corners be mitered instead of using siding fascia?
Yes, they can. However, you would have to install the siding in a way to address the inherent expansion and contraction of the boards. Therefore, the other ends of the boards (away from the corner) would have to end in a US44 butt joint fascia. The expansion and contraction would happen at this point and would be hidden through the US44 butt joint. The corners would stay in place.