Edit the classess applied to a field from outside the field template I am creating my own Panels Style Plugins. 

The styles are applied to a Panel Pane which contains a field.

From the Style, I can add a wrapper DIV to modify the display of the field.

However, I would like to be able to modify the actual class on the field itself (this would negate the need for a wrapper DIV).

In my field.tpl.php file I have placed a PHP variable for the class, but I'm not sure how I can access and modify this variable from outside the field.tpl.php file. Could some one give me some pointers. 

Thanks!
Check out Semantic Panels. It lets you choose HTML element and classes for wrapper, the field and the label. The module provides the same options that are available to customize the field output in Views.

Here's a screenshot from the user interface when you customize the field.