As I know in iOS 7 there is a new feature which allow to do exactly what you want.
However, it's only applicable for iOS 7 (and I believe it's only applicable to a supervised devices).
1) You will need to install restriction configuration profile with autonomousSingleAppModePermittedAppIDs
key (take a look here)
This is one time step and a user will need to manually accept this configuration profile.
There is another option to use MDM to install it silently, but it could be an overkill for you.
2) You should use API:
UIAccessibilityRequestGuidedAccessSession (to lock/unlock)
It is defined here
3) And most beautility part. All of it is officially document. So, you can even submit it to AppStore.
P.S. Why do you care about 4.3 or 5? They have tiny market share by now. Most likely even iOS 6 has below 20% market share already.
Update 1
Potentially, you can try to use API which sends clicks and keys (search for GSEvent in iPhone-privateapi tag). May be using these API's you can do triple home to toggle accessibility on and off.
If this work, I think you should be able to cover iOS 6. I am not sure whether both this API and triple home click were available prior that.
It looks like this private API is unaccessible anymore in iOS 7. So, probably you can ignore this idea.
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