I would like a 2 page solution on a webform.
1st page: should contain a normal webform using eg. sendmail.
2nd page: should contain a field with the "action: send-to email address" used on the 1st page.
The 2nd page will act like a kind of admin page so when the person who has the webform on his website changes the send-to email address from the 2nd page it will be effective on the first page as well..
You hopefully get the picture :)