schema.js

var schema = Joi.array().items(
  Joi.object().keys({
    a: Joi.string().valid('1', '2', '3'),
    b: Joi.string().valid('a', 'b', 'c'),
    c: Joi.object().keys({
      
      // Does this work?
      // if not, how do I reference 'a' properly?
      d: Joi.any().when('a', {
        is: '1',
        then: Joi.required(),
        otherwise: Joi.forbidden()
      })
    })
  })
)