If no other club is prepared to pick him up then he still counts under our salary cap whether we cut him or not.
If another club does want him but aren't prepared to pay his full contract (likely) then the difference still counts under our cap.
Either way, cutting Mortimer is unlikley to give us much extra room to retain/recruit players. We're better off letting go of the ones who are already coming off contract at year's end, even if they're better players than Morts.
I'm sure we could sell him to the ARU or AFL. He would be a very good promotional signing considering his a media whore.