I just called WDW to ask what the hours for Yak and Yeti would be during EMH and she really couldn't answer the question. She asked the supervisor and she said they take dinner reservations during 4:00-6:00 (???) I know that isn't right, but she finally said you'll have to pick a time when you call for your ADRs and see if the time comes up...then you'll know the hours...okay, that's not much help for planning!! 
Maybe you should choose a ficticious date that's within 90 days, an EMH evening, and ask what's available for that particular night. KWIM? If you say you're staying offsite they won't need any reservation information. Maybe they can't give hours in general because they change? I know that they can give hours if you give them a date to look at. You do not need to ask for a specific time and see what comes up. You could say "I'd like a reservation at Yak and Yeti, for dinner, on Wednesday, July whatever. Can you tell me what the latest available time is?" Or something along those lines. They should be able to. Of course it could still change for your particular dates, but at least you'd have an idea whether or not they're open now for EMH. Good luck. Getting straight answers from dining can be a chore sometimes.
