> As far as I know, there's no standard way for the composer to notate a solo for, say, violin[1][15]
Because it doesn’t make sense. You don’t know who sits there. You just want the sound of a solo violin, and the first chairs have gotten to their position because they can deliver.
There are all sorts of weird annotations in scores though, so if you think that one part sounds best when played from the last row, you can just say so in the score.