You are eligible to nominate if you have an attending or supporting membership for MidAmeriConII (the 2016 WorldCon), Sasquan (the 2015 Worldcon), or Worldcon 75 (the 2017 Worldcon) by January 31, 2015. The nomination period closes March 31, 2016 at at 11:59pm Pacific Daylight Time.
and yes, SILVER ON THE ROAD, despite being fantastical, is eligible for a Hugo.
*goes off to make my own nominations of other peoples’ books*