I have a suggestion for sorting BOM layers in a specific order across all saved outfits. I'm not entirely sure how this would work, but I propose adding a new tab in the appearance window, next to the "Outfit Gallery", "Outfits", and "Wearing" tabs. This new tab would be dedicated to BOM layers and connect to your saved outfits. The order of the BOM layers in the 'BOM tab' would set the same order in the saved outfits, as long as the BOM layers are saved in the outfits or when naming an outfit add a checkbox for the option to sync with the 'BOM tab'.
If that is not feasible, I suggest adding the ability to click-and-drag the BOM layers to sort them when in the edit outfit menu. This method would be significantly less tedious than using the up/down arrows, especially when dealing with many layers and outfits.