Yeah, this is basically the most frustrating thing ever. For me, only a handful of definitions even result in a signal on the UP2424Q: Macmini6,2; iMac14,1; and iMac14,2. Macmini6,2 is the only one that gives 60 Hz, and the other two give 30 Hz. What's even worse is that the newer definitions, Macmini7,1 and iMac15,1, don't even turn the monitor on. There's no signal at all. I don't get why that is, since real Macs with those definitions do support 4K at 60 Hz on this monitor, at least according to Apple. And of course I still can't upgrade to OS X 10.10.2 without losing the 60 Hz.
I wouldn't mind using Macmini6,2, but sometimes when I start to watch a Flash or QuickTime video, it forces the Safari tab to reload. You can just restart the video and it will play the second (or third) time, but it is super annoying.
I wish there were a way to figure out what about Macmini6,2 enables the 4K @ 60 Hz in the first place, so that maybe a separate kext or something could be written. But it's just beyond my expertise.
I guess all I can do is wait until OS X 10.10.3.