Trekkers are now permitted to make a little side trip into Upper Mustang from Kagbeni. Cross the Kali Gandaki by the bridge behind the gompa and turn right. You can then follow the clear path to the village of Tiri. I don't know if there are any facilities there where you could obtain food/drink. It's not too far so it can probably be walked in a longish morning and be back in Kagbeni for a late lunch.