You don't have to mess with the bedding at all. It's a rare occurrence for a snake to ingest it's substrate and have any issues with that. They have a digestive tract that can break down bones, fur and teeth - a tiny bit of aspen isn't going to present a lot of issues for them. If, however, you are concerned you can simply lay down a pad of newspaper over the bedding a couple of hours before you offer the snake it's dinner. Try the pre-scenting and feeding in the enclosure. Some snakes get very antsy being removed from their home and that can lead to a refusal.

We feed all our snakes in their own homes on loose bedding (other than the milksnake since her home is very "busy") and have yet to have any sort of problem.