A follow-up regarding i5 and i7 support on the C224 ASRock board: I got mine today, and in the instruction manual it says "Haswell i5/i7 processor support is an extended advantage provided by ASRock Rack. It is out of warranty, user's discretion is required." I'm not quite sure what they mean by that, and I'm using an i3 so it won't affect me anyway, but I wanted to put it out there in case it's helpful to anyone else.
Edit: Actually, reading it more closely, it specifically says the above for the C226 board. For the C224, it simply says "Intel Xeon processor E3-1200 v3 product family and Haswell i3." So they definitely appear to be saying that i5 and i7 aren't supported on the C224, and that they're "out of warranty" on the C226. Though again I don't know what that really will mean in practice.