Technically, there should be no problem using truly non-caloric sweeteners like stevia or xylitol. However, some have argued that they can provoke a preprandial insulin response, or other hunger triggers, due to conditioning. (See a good discussion on this by
Mark Sisson). What that means to me is that this could be a problem for some individuals, but not others. So I would suggest trying this yourself. If you find that it helps with the fast and perhaps works to alleviate boredom and fill the "flavor gap", then I see no harm. (I've written a post on how
flavor interacts with appetite). But if you find the non-caloric sweetners actually spike your appetite, then I would avoid them.
