From Hardi's install instrucions:
Finishing with Paint, Wallpaper or Texture
1. Painting or Wallpapering
Apply a drywall primer suitable for high-moisture areas, as recommended by the paint manufacturer. Paint HardieBacker board as you would drywall. If wallpapering, prime surface of HardieBacker board with a primer suitable for high-moisture areas as recommended by the wallpaper manufacturer.
Texture can be applied to HardieBacker board in the same way as drywall.
Use drywall joint compound, not thinset. If finishing to a smooth wall, i'd skim coat the entire board. Otherwise, i'd just finish joints between boards. You can use powdered or pre-mixed over Hardi, pre-mixed is much more user friendly. Mix any pre-mixed you use before applying and thin with a little potable water if necessary. Getting a good finish for smooth wall with setting joint compound (hot mud/quick mud/20, 40, 90 minute) is tricky to say the least. Good paint and primer properly installed, along with proper sized and used exhaust fan vented to exterior, will take care of any potential moisture issues. FYI, I've never used tile backer outside a shower due to potential moisture problems... look where i live and build.