Ended up doing this.
function getSlotAttachmentMap()
local atts = skeleton.data.defaultSkin.attachments
local slotMap = {}
for k,v in pairs(atts) do
local slotName = skeleton.slots[v[1]].data.name
if not slotMap[slotName] then slotMap[slotName] = {} end
slotMap[slotName][#slotMap[slotName] + 1] = v[2]
end
return slotMap
end