prefix_disable_hidden_fields.php

<?php
/**
 * Display hidden fields as disabled
 */
function prefix_disable_hidden_fields() { ?>

	<script>
		jQuery( document ).ready( function( $ ) {
			function disableHiddenFields(){
				$( 'form.cart' ).find( '.pewc-hidden-field' ).each( function() {
					// Display and disable hidden fields
					$( this ).show().css( { visibility: 'visible', opacity: 0.5 } ).find( '.pewc-form-field' ).prop( 'disabled', true );
				});
			}
			$( '.pewc-form-field, .pewc-radio-form-field' ).on( 'change update', function() {
				$( 'form.cart' ).find( '.pewc-item' ).each( function() {
					// Reset fields
					$( this ).css( { opacity: 1 } ).find( '.pewc-form-field' ).prop( 'disabled', false );
				});
				// Set a delay to let the plugin do its stuff first
				setTimeout( disableHiddenFields, 150 );
			});
			disableHiddenFields();
		});
	</script>

<?php }
add_action( 'pewc_after_product_fields', 'prefix_disable_hidden_fields' );