Since I seem to be doing my 'I-Spy book of West Yorkshire's stupidly steep roads' I'll consider giving it a go, but maybe when it'll be a bit less damp.
Have you tried Horsehold Road? It ends up right on the tops where it turns into bridleways so on a road bike you'd have to come back down. (Take the road over the canal at the side of Hebden Bridge Co-op, and turn right, straight onto the climb.)
You could also try the climb to Old Chamber from the back of Hebden Bridge station. Turn up by the Co-op again, but bear left and climb up through Fairfield. Continue to the end of that road where it becomes a concrete road climbing up through the woods. It gets horribly steep at the top with a cobbled section. Again, you'd have to turn and come back down because it fizzles out into farm tracks and bridleways).
There's Mytholm Steeps of course... (Church Lane left off the A646 just before the rise up to the traffic lights on the Tod side of HB.)
Also pretty tough - Birchcliffe Road. Climb that all the way to the top where the old Mount Skip pub was (now a B&B).
Cross Stone Road out of Tod. is pretty tough (with a choice of two steep finishes).
I always find the Shore Road climb from Cornholme quite hard too.
There are quite a few tough ones up from Slaithwaite, as I'm sure you know.
I haven't tackled the climb at the back of Todmorden station but that is supposed to be hard.
If you've done all of those, I could probably think of others!
Even then I was slipping all over the place. Good luck with the Impossible hills guide. 
