They don't even need to outright support it, just change to a more neutral stance, in line with their voters having different takes on it.
Labour, i can get, they have too much to lose.
Lib-dems though, surely, now is as good a time as any for them. They are far far stronger up here than down south, they are not taken as a serious contender at all, within the wider context. And their votes up here are generally divided amongst the snp and the tories, on the union question. A bit of a neutral stance and could at least drag some back based on policy.