I think the only way to this would be to subclass the EditInPlace and manually build the form to include the auto_complete stuff.
In EditInPlace, there are a couple of functions that build the form - createTextField and ... something I can't remember off the top of my head. You'd have to into these to overide the default behaviours.
Alternatively, you might be able to hack something a bit more manual. Build the auto_complete as usual in a hidden div element with a remote_form, then attach an onclick to the field you want to act as the EditInPlace ... show the auto_complete form, hide the Edit field. When the user clicks save, save the edited text and switch the fields back.
The second one is easier, but not as reusable.
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