Tried that last year and it is possible to fake it to a certain extent using the
built in GISD of TG.
You have to put anything that you want to have AO in shadows since TG's
GISD effects only show up in shadowed areas. In my case it was a single
object filling the whole image.
The object had a grey shader (lighter grey or even white would be better
though, depends on one's needs.) as in Hannes' example.
Render that aditionally to a beauty pass with your desired lighting and
combine it later on in Pshop or so.
It's a hack and might be very cumbersome to apply to a landscape shot
though, but might be worth a try.