Having experimented a little, I don’t think there’s an easy way round this. Dorico always shows the key signature at the start of a Coda, and if that key signature happens to change, it doesn’t show cancellation accidentals. This is presumably standard practice. It’s also presumably because complicated repeat structures could logically involve jumping to a Coda multiple times from different places with different key signatures.
For example, you could have
Verse 1: A Major
Coda: C Major (three naturals)
Verse 2: D Major
Jump (again) to
Coda: C Major (two naturals)
In this instance it would be nonsensical to have cautionary accidentals.
You may be able to fudge it by adding a very short extra measure at the start of the Coda, in A major, then adding the C major key signature at the start of the following bar. Removing the rest would be easy but hiding the barline would be a pain, and you’d need to do it separately in each layout.