Is it possible to display a Smart Folder as a stack in the Dock?
Instead of your specific recent documents search, you might also like this alternative:

defaults write com.apple.dock persistent-others -array-add '{ "tile-data" = { "list-type" = 1; }; "tile-type" = "recents-tile"; }'; killall Dock;


This adds a recent items stacks to your Dock that has customizabile viewing options (grid/fan/list/automatic) like one is used to.

The right-click menu gives you the option to choose between recent applications/documents/servers and favorite volumes/items.