Dorico does allow you to show accidentals above/below trills, but it’s an incomplete implementation: the plan is that the accidentals you on the ornament will affect the prevailing accidental calculation, i.e. that you can introduce or cancel an accidental on an ornament, and the notes later in the bar will take notice of that accidental. We haven’t had a chance to finish that off yet, so although you can show an accidental on the trill, it’s purely cosmetic at the moment.