I removed the reference to Whatsapp: Telegram seems more used in open communities (client is GPL), and it's already been used in the Movement for event's coordination (WMDE created a group for WmCon attendees in 2016, which is still featuring almost 90 members).
Good point about volunteers' t-shirt not being a good option. May I suggest vests instead?
If particular food options are to be requested to waiters due to their limited availability (kosher, vegan), say so on table signs, in participants' guides and whenever meal time is announced on mic.
"Don't print schedule of sessions in advance" contradicts indications at the beginning that the badge may include a schedule.
The nap room is in a certain sense a tested idea. WMDE provides bean bags at WmCon and every year has its share of aficionados, especially jet-lagged people, who don't even mind if the room is not a quiet one.