All actions used by RSTA, including those in the context menu, extend Swing's standard TextAction class, which will always operate on the last-focused text component.
You can update these actions just like any other Swing Action. Read How to Use Actions
for more information. RSTA-based actions all extend RecordableTextAction as well, which provides getters and setters for the common properties if you'd rather do that instead of using Action's putValue() method.
You might also want to check out this example
showing how to customize RSTA's context menu.