As long as they don't have to move in any way, the fact that the parts are brittle shouldn't make any difference. If they are breaking off, something is probably hitting/moving them.
I don't see anything to limit the upwards travel of your axles, other than the cast springs. It looks like the axle bearing box will hit the bottom of the cast springs if they move far enough. Any way you could put in a bump stop to limit the axle travel? That way you'd still have your suspension for the bumps in the track, and you could still use your cast springs for their good looks.
Don