Can Mountain Lion be modified so that a double click will open "hidden" folders and files?
Its looks like this might be a Mountain Lion Bug and a bug report has been submitted to Apple for this issue. Additionally, after some testing its seems that hidden files and folders will not open when double-clicked from with in a Finder window that has an icons, lists, or Cover Flow view selected.

Note:  For the Cover flow view double-click does not work in the list view, yet double clicking items in the cover flow section will cause the file or folder to open.

Note: If you double-click hidden files/folders on the desktop they open as expected, which seems odd, and kind of indicates that this not working in most Finder window views is just a bug.

Work Arounds until the bug (hopefully its a bug and not a new feature) is fixed by Apple: 

Use the "column" or "Cover Flow" Finder views.


With column view selected hidden folders just open when clicked on, and hidden files open as expected when double-clicked. 





With the Cover Flow view selected the work around is two steps.


Click on the hidden file or folder in the list on the bottom of the window.
Then double-click on the file or folder to open it.