We feed ours in their enclosures Raz, but they are on a fine aspen bedding so we've never found reason to worry about any tiny pieces being ingested. I've seen quite a few discussions here about the snake's digestion being more than strong enough for the occasional tiny bit of substrate as they regularily digest bone, teeth, hair, etc. However if you feel it's needed then by all means put down a pad of newspaper or something over the loose substrate. We did that for awhile with Rionach and Kyna till we felt more confident about this issue of substrate ingestion. Also if the prey is dry it's not as likely to have substrate sticking to it I would think.