I think it was Midge that introduced the Minimoog to the band when he joined in 1979, along with the Yamaha SS-30 String Machine
Chris started using one in the latter Systems of Romance days. Check the Old Grey Whistle Test footage from 78 on Youtube. Strangely, even though Billy and Chris were in the usual stage positions then, Chris' Mini-moog is over beside Billy.

Billy's blinding solo on On Broadway is definitely his Odyssey.