Even if the animal won't eat for, say, two weeks, there are so many tricks to use to avoid live prey. Personally, I have had to use this trick twice, once with a new BP that wasn't eating due to slow acclimation and once with a friends CRB that wouldn't eat while recovering from a bad RI, and never had any trouble getting a feeding reaction. Braining is messy and not the most pleasant thing to do, but you crack the skullcap on a f/t rat or mouse and ANY snake is going to have a very strong feed reaction.
Bloodsong