Epoxies are extremely water proof, but still I woudn't do what you're thinking.
You also have seams between the cement board panels that would need to be made water proof, and you'd need to seal well around holes made for pipes, and how you'd replace the silicone caulk around the tub in future when it's only stuck to paint would present problems. And, you very well could discover water is getting by the paint, and by the time you discover that you could end up with water damage to the walls and ceiling of the room below (if any), and wood rot on the wall studs the cement board is attached to.
(not to mention that not all cement boards are smooth, and it could look like he11 even in an attractive colour)
Why not just spend $100 on a cheap plastic tub surround and stick that up over cement board with construction adhesive and be confident it's not gonna leak? Any place that sells plumbing fixtures for renovating bathrooms will also sell plastic tub surrounds.