After compiling all of the information I've read over the past couple of days, I think I've figured out the best strategy for finding an Unborn Val'kyr. I started out flying between all of the spawn points repeatedly, switched to camping a few times, but the one thing that was effective for me was this:
1. Park 3 of your toons at each of the 3 spawn points in a particular zone. (I put 3 of my toons at each point in Borean Tundra.)
2. Cycle between the 3 toons until one spawns. A friend of mine caught one exactly on the hour at the Northern Borean tundra spawn point. So I decided to check on the hour as well. At 4am I checked all 3 spawn points and there was a Val'kyr up at one of them. The point it chooses to spawn at out of the 3 is completely random. It CAN be the same one twice in a row. Or it may not be.
It can possibly be deduced that every hour, on the hour, an Unborn Val'kyr spawns at 1 of the 3 points in each Northrend zone. By having a toon on each spawn point in one zone, you should theoretically be able to capture one within 1 hour.
If you have been spending many hours trying to get a Val'kyr, please try this strategy and see if it works for you. You can also recruit to friends to sit at the spawn points for you if you don't have 3 alts for it or you're on a server where you wouldn't have enough time to swap toons.