RE: Adding a font to the current stack (smarter fallbacks)

I suspect what you might want is something equivalent to an @font-face descriptor to define a font stack. Just for the purpose of illustration and using made-up syntax, I think you’re conceptually asking for:

@font-face {
      font-family: base-sans-serif;
      stack : MelbourneRegular, "Helvetica Neue", Helvetica, Arial, sans-serif;
}

html { font : 125%/1.3 base-sans-serif; }

#special {  font-family : LeagueGothicRegular, base-sans-serif;  }

Is that right ?

An extension to @font-face would not solve the general problem of CSS value lists redefinitions, but since fonts already have an indirection mechanism, something like this may be an option. (Though I suspect John Daggett may have 79 reasons I should be ashamed of myself for suggesting such a thing…)

Otherwise it sounds like something Variables might be able to tackle ?