"Contact Special Assistance" just means go to the Special Assistance desk at the airport check in area (like where you'd go if you had a dog). They will provide the permission there (IIRC basically a sticker that says we checked this). You won't have to provide any documentation or answer any weird questions; provided it's a normal, functional battery pack they'll just say OK. My recollection was that you had to do that for anything >100Whr, but Mark_T's post above certainly suggests that was either updated or I just remembered it wrong.