In the case of additional accidentals that warn of a key change - regardless of whether this occurs in a new line, on a new page, in a repeat, in D.S. or in D.C. - it has been standard practice for centuries, in my opinion, to place them before the bar line and not after it. However, this is not possible in Dorico. Here is a screenshot and a file.
I can’t remember exactly how I managed to do this with the screenshot. It shows how it should look. But firstly, there is too much space between the accidentals and the bar line and secondly, I can’t change the last bar line without destroying them all.
The file, on the other hand, shows a solution suggested to me by another user. Here, however, the accidentals are only placed after the bar line.
Shouldn’t Dorico provide a solution along my lines? Or does this solution already exist and I have overlooked it?
B6 Sondertusche v2.dorico (1.3 MB)
There is a brand new video – even in German – which describes how to do that (at approx. 50:00)
Is this what you want to achieve? I added a bar after the A, set D minor KS, removed the barline before the key signature, removed the rests, and made a small spacing adjustment in Engrave mode.
Cautionnary.dorico (474,0 Ko)
That solution looks great. Does it play back smoothly?
I’m afraid it creates quite a long breath before the modulation — that might be musically justified but maybe a bit too much 
I have never seen key changes notated this way. The sources I have here (Gould, Read, Ross) agree that the accidentals should go after the barline.
Gould and Ross both mention that sometimes the cancellations are placed before the barline, but not the new key signature, which is a style that Dorico does support; Gould says this happens in “old Russian editions”, and Ross says this method is “not generally seen”.
I’m curious: Can you post screenshots of this kind of notation in published scores?
I write a lot of notes that have to be played correctly straight away without rehearsal. With such scores, it is practical if everything that is notated appears logical and if the important things appear larger and the unimportant things smaller - even if traditional practice dictates otherwise.
I also do without any superfluous symbols. That’s why I use j7 instead of maj7, for example, because it contains two characters less. Furthermore, I don’t use alla-breve or 2/4 time (too much bar lines). And I prefer the notation 6/4 for 6/4 bars, instead of a notation with two bars (4/4 + 2/4), because it requires one bar line less. There’s a lot more to say on the subject.
In any case, I agree with my musicians that an additional indication of a key change before a leap should always be placed before the bar line.
As far as traditional practice on this point is concerned, I was probably wrong. That’s why I wrote in my opinion.
As a performer, I would personally find this confusing, since I am not used to seeing key signatures in this position.
I’m not trying to talk you out of your preferences – you’re entitled to notate in whatever way works for you – but I am curious as to whether there are any published scores that follow this practice.
No, the playback is not okay. But that’s not a bad thing, because all that matters to me is that it looks good.
No, you don’t need a long breath for that. Maybe you didn’t look at it properly.
Sorry, I was joking! I didn’t look at it at all, and I guess that one whole 3/4 bar of breath here is anyway too much!