You may have to wait for someone from Canada to respond...not sure Dumapan Panels are available in the USA...never heard of them, but that isn't definitive!
Cement board is designed to have tile on it, not for paint. At least in the US, outside of the shower, 'normal' drywall is typically used. If you don't have proper venting, the paint may not hold up well on any surface, regardless of the material, but inside of a shower is a totally different issue, as there is liquid water and pressure you do not want in the walls. Densshield has fiberglass exterior and can be painted or tiled. It will work, but there will be some texture.
A semigloss or gloss paint is more robust if you have condensation in the bathroom, but again, good ventilation is important.