http://docs.cksource.com/CKEditor_3.x/D ... le_Browser
window.opener.CKEDITOR.tools.callFunction( funcNum, fileUrl, function( alt ) {
// Get the reference to a dialog window.
var element, dialog = this.getDialog();
// Check if this is the Image dialog window.
if (dialog.getName() == 'image') {
// Get the reference to a text field that holds the "alt" attribute.
element = dialog.getContentElement( 'info', 'txtAlt' );
// Assign the new value.
if ( element )
element.setValue( alt );
}
});

Re: Example4 of Integrating CKEditor with a Custom File Brow
So in your custom browser should have:
function alt4Filebrowser(alt) { return function() { // Get the reference to a dialog window. var element, dialog = this.getDialog(); // Check if this is the Image dialog window. if (dialog.getName() == 'image') { // Get the reference to a text field that holds the "alt" attribute. element = dialog.getContentElement( 'info', 'txtAlt' ); // Assign the new value. if ( element ) element.setValue(alt); } } }