Version 0.4 brings a couple of new features to the Select Within plugin.
Most importantly it now supports mostly within and percentage within selections.
Also introduced is Pole of Inaccessibility within selections. As implemented in the QGIS core with the Polylabel algorithm: https://github.com/mapbox/polylabel
Point within selection examples. Centroid (red), Point on Surface (green), and Pole of Inaccessibility (blue):
Point on Surface within:
Pole of Inaccessibility within (1.0 tolerance):
GitHub Code Base
Issues and bug tracker: