The Trash KCM affects the whole workspace and not just dolphin. Currently it is embedded in the dolphin settings dialog, while it is not reachable from Plasma's systemsettings.
At the systemsettings BoF at Akademy it was decided that we don't want to move it to systemsettings (that is already quite crowded). What we could do instead is make Dolphin and Folder View show a simple "Configure Trash" button that opens the Trash KCM in its own window.