The Rectangular Select Tool with Rounded corners option checked is very useful, but it could be so much more. The underlying GIMP function, gimp-image-select-round-rectangle written by Martin Nordholts provides much more functionality than what is exposed in the Rectangular Select Tool.
This script attempts to unleash gimp-image-select-round-rectangle's potential by raising the radius limit to 262144 pixels from 100 pixels and exposes the ability to specify different x and y radii.
In case you are wondering, a bug report https://bugzilla.gnome.org/show_bug.cgi?id=589473 has been submitted to "fix" the Rectangular Select Tool.
This script is also located at: https://github.com/m2orris/GIMP-scripts
This script contains two functions.
Select --> Rectangle with circular corners ...
Select --> Rectangle with elliptical corners ...