OK, I guess it wasn't the way I expected to solve my problem (hence it's kind of off-topic from the title) but I figured out a way to do what I intended to do in the first place.
I went to Preferences -> Folders -> Folder Formats and changed the default "Flat View" format to what I preferred. Then I set a hotkey to "Set FLATVIEW=Toggle," and it works pretty well.
The only issue I have is that in the Flat View, it still shows some columns (Modified, Attr, Type) that I specifically removed from the Flat View default format (in Flat View - Edit Format -> Columns listview). What am I doing wrong? Is it inheriting a setting from elsewhere?
Also, I've gathered that my original question is probably not possible right now in DOpus 9, but it would be a nice feature.