Knowing you have a slate roof, I'd be extra careful about where I vented that fan because a roof tear-down of a slate roof isn't done with spare change. I'm not saying that the roof will fall in on you, but I personally feel that any roof is only as good as the structure and materials used underneath it, so creating a mould zone doesn't turn my crank. I am sure guys like Ed the Roofer on the Roofing forum could tell you more about that, but to me it's just not a good idea.
So if you still don't want to insert a vent in the slate, what about the soffits? Seems to me like doing something like that is actually killing two birds with one stone: humidity problem and extending the lifetime of your roof.
Look, at worst, the cost of installing a vent in the soffit might run you a half-day's pay for two guys...say $500. How would that compare to a roof job? I guess it depends on how long you want to stay in that house, but also consider the resale value of your house if you do nothing.