Check it out! The proboscis action is in the bottom right corner of the video. Each proboscis is a beige trunk-looking thing with a pink tip. Look for them moving inside the open mussel. The GoPro is not exactly in focus, but you can still see each proboscis fairly well.
I am starting to realize that perhaps the best way to get a dogwhelk to feed is to make it jealous; once one dogwhelk is drilling, others tend to join it. There was another healthy mussel in the tank with these dogwhelks and they ignored it and all fought over the same meal! How strange. Granted the other healthy mussel was glued to a hydrophone, but I have no reason to believe that would deter the snails.
Speaking of hydrophones, mine has been picking up lots of really loud static. I have checked the connections and moved things around and nothing seems to make it better or worse. It will randomly go quiet sometimes, but today it was very static-y for hours. If you have any recording experience and think you can help out, please let me know! The only thing I can think of now is maybe being in seawater continuously for several days is having a negative effect on it.