Use Alternate-Protocol header to advertise SPDY to clients not using it already.

spdy_alternate_protocol.lua

-- the nginx variable $http_x_spdy_protocol contains the value for X-SPDY-Protocol
local SPDY_REQ_HEADER_VAR='http_x_spdy_protocol'

local ALTERNATE_HEADER='Alternate-Protocol'
local ALTERNATE_VALUE='443:npn-spdy/2'

local function is_spdy_on()
   local spdy_version = tonumber(ngx.var[SPDY_REQ_HEADER_VAR]) or 0

   ngx.log(ngx.DEBUG, 'SPDY Version: ', spdy_version)

   return spdy_version ~= 0
end

-- Use Alternate-Protocol header to advertise SPDY
function advertise()

   if not is_spdy_on() then

      ngx.log(ngx.DEBUG, 'Advertising SPDY')

      ngx.header[ALTERNATE_HEADER] = ALTERNATE_VALUE
   end
end