This has nothing to do about episode 70 (well, sorta, as Hanatarou is in this episode)
Call me crazy, but I believe that Hanatarou may very well be an agent of Aizen's. Why?
1. Hanatarou was in charge of cleaning Rukia's cell before she was moved to the Tower. Yes, menial jobs like this are usually given to the 4th squad. But Aizen was already in control of Central 46 and could have specifically picked anyone he wanted to keep an eye on his objective.
I see assigning Hanatarou to clean Rukia's cell as a way for Aizen to gain information about what happened in the outside world. Rukia wasn't going to talk about Ichigo and the events outside to just anyone. Someone needed to befriend her, someone who she would see everyday and gain a friendship with, and then she would divulge information. This info would be critical because the people Rukia would talk about would probably be involved in the rescue attempt.
2. Hanatarou helps Ichigo and Ganju.
I never accepted Hanatarou's proclaimed reason for aiding them. I know that he's supposed to embody the 4th Division and their benevolent nature, but for the guy to go and betray the Gotei 13 for a person he's known for only a few weeks? At that point in the story there wasn't any real division in SS over the execution. Even Renji, who loves Rukia, wasn't yet ready to go against SS. People were like 'Yeah, I don't like it', but none of them were ready to go against it, not even Shunsui and Ukitake.
The Ryoka were, as Aizen said "a diversion". But at the point Hanatarou joined them they'd only really defeated Ikkaku and some grunt Shinigamis. No biggie. Then Ichigo beat Renji. Aizen even described this as the event that made the Gotei 13 get really serious and focused on the Ryoka. Its at this time that Hanatarou heals Ichigo, making sure that the 'diversion' can continue.
Hanatarou also guides Ichigo towards the place where Rukia is being held. He shows them the path in the sewers so they can get to the Tower (and create a bigger ruckus), rather than get bogged down in fighting more grunts.
Hanatarou also gives a knockout pill to the guards at Rukia's chamber. I don't know how strong these guys were supposed to be, but if they're in charge of guarding a jail when enemies as strong as Ichigo and crew are roaming about, they probably weren't pushovers. Only a recognized Shinigami could get close enough (and only a medic/errand boy like a 4th Squad member) and be trusted by the guards, avoiding another meaningless battle. At this time Hanatarou also produces the keys to Rukia's cell. Did he steal them off the guards? I don't know, but if not it means that he'd grabbed the keys at an earlier time...
3. Hanatarou "got seperated from his group and stumbled into Ichigo and Ganju"
I don't buy that shit for a second. The guy stops to tie his shoelace, gives his comrades (who don't really like him) the chance to leave him behind. Then he 'wanders around' until he bumps into Ichigo and Ganju about to fight an army of 11th Division. Hanatarou 'stumbles' and while trying to regain control knocks down a large number of the 11th squad guys, pushing his way towards Ichigo and Ganju, and suddenly laying at their feet, all helpless and naive, making himself an irresistable hostage.
4. The conversation between Hanatarou, Ganju and Ichigo when he's 'taken hostage'
After they escape the 11th Squad Ichigo and Ganju are debating what to do. They want to leave Hanatarou there.
But Hanatarou simply exudes wanting to help them from the first second. He introduces himself to them, so polite and friendly. He goes out of his way to tell them where Rukia can be found. He also volunteers to lead them there, via a secret route that only he can navigate. More importantly, (and one of the reasons why they decide to take him along) is that Hanatarou keeps emphasizing how weak, picked-on and friendly he is. Ichigo and Ganju decide that 'he can't be any real threat'.
If Aizen wanted to learn more about Ichigo (the main threat as revealed by Rukia's stories) then Hanatarou would HAVE to go to lengths to stay with them all.
5. Aizen, Gin and Tousen have all left SS. Do you really think Aizen's stupid enough to not leave behind a spy? He's going to lay low for an entire year, without keeping tabs on what's going on in SS? The man's around proven to us that he is not that stupid.
6. It's nice to have a friend who can bring you back to life. No, seriously. A healer would be a great asset to any faction, Aizen's included.
I know this theory is a little wild. Maybe Hanatarou just is a super-nice, super-meek, super-naive, super-weak, super-clumsy, super-social retard with a heart of gold. Maybe he really did want to save Rukia, whose 'voice put me at ease'.
Or maybe he's the best spy you could ever want, the last person anyone would suspect.