One easy workaround for this (on DW) is for the poster to set the entry to NSFW/14+ -- the entry will then display an are-you-really-sure-you-want-to-click-this interstitial, which has the option for people to specify why the entry is so flagged, so you can say "this contains spoilers for ..."

It does rely on the person posting the entry, not the person linking the entry, but it's a workaround until we can figure out something to do what you're asking for here!