But can you imagine a Soviet court declaring a law to be in violation of human rights?

Yes, much like the EU, they would regularly over-ride the ‘opinions’ of their subordinate states.

The central party and state organs in Moscow would sometimes overrule decisions by the governments of the SSRs and other subordinate entities. But they didn't do this by having the Supreme Court of the Soviet Union declare laws unconstitutional. They did it by administrative fiat.