Providing the method name as the first parameter is a bit awkward,
perhaps looking at the alternatives would help:

Add namespaces to jQuery.  This isn't very jQuery-like.  Example: $
(el).tabs.add(url, label).show();

Add a new jQuery method for every plugin instance method.  This
pollutes the jQuery namespace, so this should only be done when it
really makes sense.  Example: $(el).addTab(url, label);

Use events.  You can bind custom events in a namespace and then have
users interact with your plugin by triggering those events.  Example: $
(el).trigger('add.tabs', url, label);

There may be other approaches as well.

The jQuery UI approach allows plugins to expose as many methods as
they want while only using one method in the jQuery namespace.