Here are two ways that I think get you what you want using current functionality:
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If the relevant binder hierarchy is exploded out (i.e. no collapsed folders), then selecting everything there will make the Outline view (duly sorted) do what you want. Each item (doc or folder) will be flat-listed and sorted according to its own mod date.
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Doing a project search or using a Smart Collection will give you a flat binder list of docs. So, if you can catch everything you want with a Smart Collection, then you can invoke that, select all the resulting docs in the binder and your outline will once again give you what you want.
-gr