I was able to figure it out. So for future reference...
Near the top of the module_script, is some lines where you need to add in items not already in multiplayer, for them to show up. Looks like this:
#arrows
(item_set_slot, "itm_arrows", slot_item_multiplayer_item_class, multi_item_class_type_arrow),
(item_set_slot, "itm_barbed_arrows", slot_item_multiplayer_item_class, multi_item_class_type_arrow),
(item_set_slot, "itm_bodkin_arrows", slot_item_multiplayer_item_class, multi_item_class_type_arrow),
(item_set_slot, "itm_khergit_arrows", slot_item_multiplayer_item_class, multi_item_class_type_arrow),
And you can add your item line at the bottom of those or anywhere along that area. I made mine in a group with #custom at the bottom of the "item_set_slot" lines, so they'll be easy to find.