Dorico can put a key signature change at any rhythmic position by activating the caret, moving it to that rhythmic position, typing Shift-K to open the key signature popover and entering the desired key. But in your example, if the key signature is changed to D minor on the last 32nd of bar 29, the change to C major in bar 30 will not have the correct number of naturals to show the change from C minor:
What we need is a key signature that looks like D minor but behaves like C minor. To do this, edit the 12-EDO tonality system, add an accidental that looks like a natural but behaves like a flat by setting its pitch delta to -1, and add a custom key signature that has a normal Bb and the new accidental in place of Eb and Ab. Now activate the caret, move it to the last 32nd of bar 29 and change to the custom key signature. This provides the correct number of naturals in bar 30 but has too many at the end of bar 29:
The final step is to edit the tonality system again and delete the natural glyph from the new accidental:
