The problem with setting up Spellcast to only handle equipping Dusk/Koga is that it's going to process those changes whenever anything happens. So during the day whenever you perform a WS, Spellcast will equip Dusk in your hand slot over whatever you equip there. Your best bet is probably to use a pre-created Ninja xml that handles day/night gear and just replace the gear listed there with what you have and get rid of gear macros altogether, Spellcast works just as well for Melee JAs/WSs as it does for Mage's Spells.
Here's a link Silvenmysts NIN xml:
Silvenmyst's Ninja v2.17 - Windower Wiki. It will work with the version of Spellcast you can download from the Windower site (2.22) but not the 2.30 beta included in this thread without using Aikar's XML converter on it.