It was permitted/possible during the 2006 and 2007 Free Dining Promotion periods - due to SO many Guests having the Dining Plan but not being able to get into any Table Service restaurants.
Otherwise, officially no - but it may be at the discretion of the Resort Manager (i.e. it's likely impossible at an in-park location).
I did it in 2007 at Pop. You just told the the cashier to ring it up as a TS and she gave you a coupon good for 2 snacks but you could only use them at Pop.