Incidentally, it looks to me like you should be able to test macroscopic decoherence. Eventually. You just need nanotechnological precision, very low temperatures, and perhaps a clear area of interstellar (intergalactic?) space.

Short of that, building a scalable quantum computer would be another (possibly easier!) way to experiment with macroscopic coherence. The difference is that with quantum computing, you wouldn't even try to isolate a quantum system perfectly from its environment. Instead you'd use really clever error-correction to encode quantum information in nonlocal degrees of freedom, in such a way that it can survive the decoherence of (say) any 1% of the qubits.