My thoughts...
I would be skeptical.
1. If the paintballs come apart before leaving the barrel, you might wind up with a mess that is impossible to completely eliminate.
2. I like the idea of a sabot but I think I would use a patch instead. In this way the patch cleans the bore and (I would think) reduces the likelihood of the paint or the plastic from the ball fouling the bore.
3. Is either the paint or the plastic ball flammable?
4. What is the MV of a paintball fired from a CO2 platform in comparison to one fired from a weapon intended for use with lead balls? More appropriately, can the MV be kept low enough that the paintball does not simply come apart as it leaves the barrel?
5. I think I would start with smooth bore weapon rather than one with rifling. Easier to clean, Less chance of the rifling cutting the plastic.
Carrying this idea to the next level... Now don't laugh....
How about building a very small field gun which would fire a projectile made of about a dozen paintballs wrapped in a package, ala "grapeshot"? I am thinking of something that looks a good bit like a mortar.
The paintballs would be wrapped in saran wrap or some such. The seal round the projectile would have to be very good so as not to let the expanding gas escape between the balls.
The mortar would be made of inch or inch and a half plumbing fittings.
I am not kidding. I think this could work and could be done safely as long as the weapon is not over loaded.