Well, FWIW you have mine - I think it's an excellent idea! If you create the comm then those that want it will join ("if you build it, they will come" *g*). Then it's up to the other comms to decide if they are going to adopt that method of referencing.

I have the tiniest niggle that existence of a reference to a 'spoiler' comm might constitute a spoiler in itself, but if the scope of entries at the comm is broad enough, specific 'warnings' should disappear into the mix. For example I don't think anyone blinks twice at the Hatstand announcements that a story appears in a story list (cos these included various types of AU, BDSM, Christmas stories, crossovers as well as the more typically 'warned' for death or rape).

Of course this might not be exactly what you were thinking...