I believe it will be a no-no. The whole point is to have everything already broken down to the very basic levels, so that your poor gut won't have to do much work. I think you'll find that tea will be out, whether herbal or caffeinated.
My personal opinion is that the dietitians are used to dealing with a bunch of whingers. (Though this may be unfair). Honestly, my enteral feed wasn't / isn't bad at all. It just tastes like moccha, but with a slightly metalic edge.
Some people find that it tastes better refrigerated - bear this in mind for future reference.
Anything I say, please bear in mind that I did mine through a naso-gastric tube, and only started drinking it straight once the tube came out (after about
six weeks), as I was slowly reintroducing other foods.
I honestly think you should go for it. And if you do it now, before the warm weather hits, you won't need to miss out on yummy things like ice cream. You'll even have it over in time for Easter, so you can eat chocolate.
Think of it as a really *good*, meaningful, Lenten experience...