No, sorry.
Think that would make sense only if there would be a regular tessellated mesh (as the default Plane object).
With the SELECT brush you can only select full triangles which are (fully) inside a certain brush radius.
If you need to select a certain shape (as a square) use the STAMP tool to draw the shape (this splits the triangles hit by its boundary) and select the resulting group by double click.
Gunter Weber
Triangle Artisan
