Rep/loot in Hyjal?

There are indeed protections put in place that should make it extremely difficult for very small groups of players to activate the Battle of Mount Hyjal event and be rewarded with either reputation or items from killing the incoming enemy creatures before the players suffer a defeat. Whenever an enemy wave spawns, there's a check done to see if you've killed enough of the enemy creatures. If you haven't, then loot/rep is disabled for that wave of spawns. If you have killed enough, then that wave and further waves of creatures leading up to the next boss have loot/reputation.

Under relatively normal conditions where a full raid is attempting to fight through the enemy waves of creatures to defeat a boss, all creatures past the first wave should reward reputation and have a chance at item loot.

These kinds of protections were necessary so as to not promote the creation and activation of large numbers of Hyjal instances, which could lead to performance problems for the server.