The UNCITRAL 2008 and 2012 Digests do not report any case law applications of Art. 20.
The following case mentions Art. 20 but only in a passing reference:
Austria 24 February 1999 Oberlandesgericht [Appellate Court] Graz (Military weapons case) [translation available]
The following additional reference to Art. 20 in case law is provided:
Netherlands 19 August 2015 Rechtbank Den Haag (Silicon Refractory Anchoring Systems B.V. v. Refrattari Sirc S.R.L.) 20A