Idea #1: "Tripwire": If any of  words exist in the diff versus the downstream prior to the merge from upstream, then flag the need for manual review.
Example tripwires:
  UNCHECKED_BITSTREAM_READER
  SPDIF_DEMUXER
  etc

Idea #2: "Auto-check downstream patches": Consider reworking the patches/README into something automatically verifiable (like a checked-into-downstream-patches-diff-file that, once reverted on top of the merge-from-upstream result, should result in no diff versus upstream). (Co-credit to liberato@ for this latter idea.)