maxspeed=none implies there is legally no speed limit (Germany, Isle of Man). I am not convinced it should be used in this circumstance. The legal status around unadopted roads is very blurred, hence why I am of the opinion that tags should be left out (also the fact they are called unadopted roads in the UK meant I've been using the tag "unadopted=yes", but if you want to set a new standard for that, go ahead as there is no official tag).