Form submit from modal window to parent window

Use the parent method to pass data back to your meta field.

Create a JS function to handle the passing of data from Thickbox to the meta field:

function foo_interstitial(data){
   $('.form-field-selector').val(data);
}

Add a submit handler to your thickbox instance that passes to the parent.foo_interstitial() function.

$('.thickbox-submit-selector').submit(function(){
    var _value_to_pass = $('.field-to-pass-selector').val();
    parent.foo_interstitial(_value_to_pass);
});