Reply To: GREP Style with Search/Replace?

I haven't fully tested, but one work around might be to set a Character style with a large amount of tracking (you can't set kerning in a chara style) and apply that to the em dash AND the letter preceeding it. Seems the tracking only applies to the right side of the selected character, so the preceeding character needs it applied as well to get equal spacing on both sides of the em dash. You should be able to set teh GREP style to apply to the emdash and preceeding character pretty easily.

I had to do a similar work around for TM followed by a comma or period. Grammatically it's correct to have the puncuation follow the TM, but visually it looks wrong. I created a chara style that changes the tracking for the TM/comma/period pair that moved them together for a better visual pair. The only “gotcha” was that to make it work, the puncuation has to be placed before the TM. So I have to run a find replace to swap those, but at least they're easy to spot in copy when I forget to replace them

Good luck!