There are TWO seperate unlocks. actually, three...
First one is on the OTHER tab. This is required to make him available legacy wide.
Second/Third are on Caracter Perks. One is for Republic Toons, the other for Imperials.
These will unlock him on individual characters. Be careful you don't unlock the wrong faction, you won't get a refund OR droid. You can unlock him at any level. I got him at level 4 for my bounty hunter.
Does this mean that to unlock him for your Imperial 'toons you must first complete the HK 51 questline on a level 50 Imperial? That is, as I completed it on a Republic toon, he doesn't become available to unlock for my imperial toons, only my Republic ones?