Pretty simple.
Make the spline paint a specific material on the landscape.
Then use the procedural tools to spawn the correct type of foliage onto that specific landscape layer.
Painting is additive, so if you make your spline controls very wide, paint with a nice falloff, then reduce the spline points and paint again with a different material you can get several layers done.
Obviously this compromises landscape layer complexity and has a chance of running out of samples.