Edit returned result items

I'm working on making an extension that will enable a user to edit items in a text file. What I'd like to do is have returned results enable a "handle_string()" handler, but pre-populate the handled string with the returned result. Example:

Returned items:

Line 1: Foo bar
Line 2: Bar Baz

Select line 2 item, click space bar, and have the field pre-populated with "Bar Baz".

Is this possible? I can't seem to figure out how to do this in the API.

Thanks,

Nik