I would like it if the custom thunar actions were generated dynamically by the union of a global root-editable file (/etc/xdg/Thunar/uca.xml) and a local user-editable file (~/.config/Thunar/uca.xml)

Possible conflicts resolution:
Make the "name" field be unique.
If a local uca.xml action has the same name as the global one the local overrides it.

Editing global actions:
Either
  A)  disallow editing of actions in the global file or
  B)  upon editing of a global action copy the action elements from global uca.xml to local uca.xml.  From there on in, this action will override the global one (from above)