knight504 said:
Question: Why was it necessary to hide the Hogyoku inside Rukia? Why not stick in some unrelated human (altering their memories afterwards) instead of trying to turn a shinigami into one?
Urahara hid the Hogyoku inside Rukia before he left SS, back when she was just a random urchin. Presumably, she was supposed to live and die in Rukongai, a form of security by obscurity, and anyone coming after Urahara or his associates for it would be out of luck.
... or at least, that's how I've always interpreted things.
Aizen tells Ichigo that he had been looking for where Urahara hid the Hogyoku, and that by the time he discovered that it had been hidden in Rukia, <a href='http://www.mangahere.com/manga/bleach/v20/c175/16.html' target='_blank' rel='nofollow'>she had already disappeared</a>. Aizen says that he knew intuitively that Rukia disappearing was Urahara's doing.
None of that makes any sense unless you assume that the Hogyoku was hidden in Rukia all along, rather than being done after she came to Karakura. It would be a little odd to say that Urahara tried to destroy the Hogyoku, couldn't, and then hid it in desperation, if he actually held on to it for a century and then just hid it recently.