Would you use it to cover a cake board?

Yes.

  I think it should be fine for covering the cake board.  How expired is it?

I also use old fondant on cake dummies. 

AI've just checked my tub of Satinice gumpaste (and it's out of date!). It's a best before date, not a use by date so it's definitely ok to cover a board. Best before means just what it says, see what it looks like, smells like, feels like and use your own judgement.