I might be a bit naive but I don't think anyone would take advantage of this. I mean yeah, it could get you new readers by getting more often into the fresh section but how many would you loose when they get fed up getting all those "fake updates" notifications ? Readers don't like cheaters =)
And maybe adding a button like the "bell" to specify if you want it to be considered an update... that way, for the rare cases you're speaking of, the author could choose not to consider it an update if he just divided an existing file... I don't know...