There are also Sweep native plugins which do things LADSPA plugins cannot, such as reversing and normalising the selected regions, and modifying the boundaries of the selection.

For example, the Select by energy function works as a plugin that creates a complex selection.

Parameter dialogs and undo/redo functions are handled automatically, so plugins are quite easy to write. If you want to write a plugin there is an extensive Plugin Writers Guide documenting how to go about it. This is available on the web site and is part of the source distribution.