I have a working solution.

1. In the Developer Tools Console, add the following:

   var userTemp = atom.commands.add('atom-text-editor', {
   	'user:temp': event => {
   		var editor = atom.workspace.getActiveTextEditor();
   		editor.selections.forEach((s) => {
   			s.insertText(`[${s.getText()}]`);
   		});
   	}
   });

   This creates a new command called user:temp

2. In keymaps.cson add:

   #	macOS
   	'.platform-darwin atom-text-editor:not([mini])':
   		'cmd-A': 'user:temp'
   #	Windows
   	'.platform-win32 atom-text-editor:not([mini])':
   		'ctrl-A': 'user:temp'

   This assigns cmd/ctrl-shift-a to user:temp

3. If you need to replace the command, first dispose of the old one:

   userTemp.dispose();

The only downside is the last step. I can’t work out how to dispose of an old command once you’ve lost track of the disposable variable.