In this update, the script will work on sub-object selections. Also, a bug in the first version is fixed, when you switch to another mode other than the Modify Panel, the first version gives an error, this is fixed now in this 1.5 version.
To recap on why this plugin:
The default Zoom Extents Selected tool in Max has an issue when you're in a sub-object level but no sub-object element is selected, pressing Z(the default shortcut) causes Max to Zoom Extents on the whole scene instead of the selected object.