What was that air shotgun that was around in the late 90's?
I think it used a plastic sabo to contain the shot.

What you have there should work but I can see a couple of possible problems:
Depending on the chalk mix if it disintegrates too fast then your "round" will present very little resistance in the bore, which could be a problem for a springer.
The chalk may clog up the rifling grooves.
Since a single well fitting pellet is not allowed above 12ftlbs then it follows that if you have ten projectiles then each can have no more than 1.2ftlbs, minus the power lost accelerating the mass of the chalk. I'd have a weigh of the finished projectile and then weigh the lead shot on their own and you can work out the weight of the chalk.

But do let us know how you get on - it's a fun project and it'll be interesting to see the results on a pop can at short range!