Sedation slows down all the bodily functions so using it on a full digestive system may result in impaction. There is also a chance of choke when feeding hay. I never feed before sedation and also leave a good few hours after they have 'come round' before feeding hay or bucket feed.
You can feed right up to and after sedation, it makes no difference as to strength etc as it is straight in to the blood. You just have to watch out as reactions are slower but mine eat when sedated with no probs!
Luckily our clydie is good to clip - something that size not co-operating is not fun.
We have to be very careful with Cairo when sedating - he may be a big boy but he only needs the amount of a small pony - hence big notices by his stable to only administer tiny amounts in an emergency.