i don’t think i’ll ever integrate this particular thing into the plugin (after all, don’t you pay the tip when you get the stuff delivered ?)

however, you can try the following:
copy templates/wppizza-order.php to your theme directory and edit/add along the following lines (these will get included in any/all emails send):

/***********************************************/
//if you want to add custom  field value pairs, make sure
//its an array having [label] and [value] to display in email
//i.e:
<input type="hidden" name="distinct_name[label]" value="some value"/>';
<input type="text" name="distinct_name[value]" value="some value"/>';
/***********************************************/

hope that makes sense. (shout if not)