You can feed them immediately after shedding, as long as they got it all off. Really it is the eyecaps that matters, I think, because I think the reason they are reluctant to eat during shed is due to reduced visibility. As Pit on the Prowl said, you CAN feed them even in deep shed. I do all the time. Some eat, some don't. But, since your snake is already refusing, I'd wait until after the shed to reduce the number of complications.
Side note: The above applies to BPs only. I also have corn snakes, and they appear to have an increased incidence of regurges if they eat during shed. I've never seen nor heard anything to indicate BPs have that problem though.