Agreed, I also find it annoying to shell out 1,000,000 credits with every character once he is unlocked. I would like to see HK-51 as a Legacy bound unlock like Treek, T4-1D, and Darth Hexid to name a few are. That way you can pay X number of credits, or X number of cartel coins and be able to account wide unlock HK-51 where all you have to do is get an item via mail or collect it in your Cartel Collections screen. Being that HK-51 is the ONLY companion in the Legacy option to unlock via a million credits, it kind of seems antiquated now to have to spend that many credits per character in order to unlock him once you've done the HK-51 Section X quest on a character, well technically two since you needed two characters one at at LEAST level 50, and one on the opposite faction at at LEAST 15 to collect them all then to go back to Section X and complete said quest. There's people out there that don't milk creds like crazy like others do, whereas there are people that can shell out a million like it's nothing....
However yes, I would say that ONCE you've completed Section X and obtained HK-51 once, they should be capable of sending you a contract, or summon holo for the droid much like they do for say Shae Vizla, Master Ranos, and Darth Hexid to name a few. OR Make it so that you need to go to the fleet at level ten and obtain him via a Cartel Bazaar vendor once you have the "contract" for him. Either way it would make obtaining something that you've already got the Legacy unlock for, that much easier and a more enjoyable experience at least in this player's opinion.