You can of course be sued for absolutely anything, but whether you can be successfully sued is another kettle of fish.
Whilst I can see the argument against putting things like "Warning loose surface" on a routesheet, but I can also see that you'd feel pretty bad if you choose not to put a warning for the reasons stated, and someone did come off on a bad surface.
I would have thought that you could add a coverall along the lines of "indications of road conditions on this routesheet are only advisory and do not preclude actual conditions differing, or not being indicated", and of course the age old "Use at your own risk".
You have to behave sensibly, and assume that most other people will (including the courts), otherwise you wouldn't get out of bed in the morning for fear of being sued, and your certainly wouldn't get involved in organising any cycling events.