How Plasma handles the Meta key is kinda special, so instead of going to kglobalshortcutsrc it goes to kwinrc instead.
FWIW I did this at some point, a small Kdialog window that shows up asking if you want to enable/disable using Meta to open KRunner. Internally it’s just the snippets of code you already used there. For it to work you need to know if your distro provides qdbus or qdbus-qt5 or qdbus6.
I need to check if it still works (it probably does).