This is particularly useful involving situations where the trill is to a microtonal interval, to the same note (colour/enharmonic trills), or for double trills in keyboard music.
Can be faked at the moment—I use a grace note sized notehead, which seems to be standard, and add parentheses as text objects—but would be nice to have as a default option. The potential functionality seems to be there as it’s already possible to set the interval above/below in properties.