I don't see a problem getting in at 5am. Even if there is a queue, I don't think it will be that long. As seacarl noted, you likely won't be at an agent at exactly 5am anyway, so it will probably work out. If not, and you're on one itinerary, I bet the layover will count even if overnight.
Just FYI, you'll still only get a continental breakfast the first hour you're there. Hot food doesn't start until an hour after open. You should still have a solid hour to enjoy it, though.
For arriving, if you're seated in front, get off quickly, and aren't at the far end of B, maybe you could get there in time to grab a very quick bite (it's in C and will require a train ride). I think they'd let you in, but honestly not sure as I've never had a layover like that where I've tried it. Another option, if you just want free food, is the SweetWater Taphouse in B (where you'll fly into) which is a priority pass restaurant and they're open, I believe, until 10. I don't think they've ever checked my boarding pass, but I think you're ok regardless. You'll know right away when you check-in.