No, Niagara can only sample the mesh surface or distance field for now, so there is no easy way to uniformly spawn inside a mesh.
You can sort of hack it together yourself with the above approach. Another possibility would be to do some rejection sampling, just spawn particles in a grid, then each particle checks if it’s inside the mesh or not - but to get narrow features like the spider legs with this you would need to spawn a lot of particles, so it always depends on your use case.