@ccstone, thanks for your answer.
I hadn’t thought of using a script, and it is indeed a good idea. The only point I’m struggling with now is how to pass the filename (and path) to the script. The move action is contained in a for each action, that iterates through the finder selection. I saw another discussion about a similar issue, Processing selected files in a python script, but I’m not sure I understand it that well. Besides, there the “execute shell action” calls a python script already on disc, whereas here it would be just a single command.
The solution in the original post works, I just feel it is a little too kludgy, and was looking for something more elegant. Your alternative fits the bill perfectly.