While browsing through a Lortzing piano reduction I came across a real world example of what Kentaro is referring to. Context (the tempo of the passage is rather slow) makes it quite clear that this is its own symbol, of fixed width and relating to the start of the note and not to the note's entire duration. Therefore, "messa di voce" is not meant by that symbol, otherwise the whole passage would be flipped to its opposite: the swelling sustained notes would drown out the remaining three voices (which are obviously designed to make the lyrics well understandable). In reality, the sustained voices are supposed to stay at their general soft dynamic level, but to have a... yeah, well... a kind of... soft accent... or something... at the beginning.
Alex
