Hydrogen cyanide - a Lewis acid?
Low-lying, empty antibonding orbitals will suffice.  The reaction of carbon dioxide and water to form carbonic acid serves as a parallel example.  Water donates one of the lone pairs on oxygen towards bond formation, water is the Lewis base in this case.  Carbon dioxide has empty, low-lying $\ce{\pi}$* orbitals that serve to accept these electrons; carbon dioxide is a Lewis acid in this example.