first, i would prefer to use a new indicator in the fringe, when the
fringe is present - seems like this would be a nice use of that real
estate, more suitable than precious minibuffer content space. the new
cue would display with the fringe top-of-buffer signal, when people
use that, but that's generally redundant in the minibuffer, since it
always has a prompt on the first line, and usually contains one or
just a few lines.
I and, I think, many others, turn fringe off. Hate it; don't need it.
I think one or two characters of minibuffer space is not too much to
sacrifice for this feature, and it would be optional, in any case.