Ingesting a little bit of substrate is not a big deal. If you are particularly worried, you can use paper towels or newspaper.

Or you could lay down some paper towels or newspaper prior to feeding.

The argument that feeding outside the enclosure will teach your snake to not associate your hand with feeding makes no sense. Feeding outside the enclosure to reduce substrate ingestion is fine but it really is not a big deal to ingest substrate from time to time. They rarely get impacted.