Several local ribsters (and myself occasionally) have regularly used power washers for cleaning ribs when lifted out of the water after sitting in the local marina for several weeks / months and I am not aware of any problems being caused.
By the way, have you (or anyone else) heard of the Johnsons Baby Lotion treatment? One of the local guys was told by a local Rib builder to try using baby lotion instead of antifouling, particularly if the rib is lifted out for power cleaning several times a year. Whilst it (is claimed) it doesnt stop marine growth, it does make it much easier to get off - even a fast run cleans most of it..
I have found that pressure washing tubes is almost useless, but does no harm! I suppose that if you have a seam that is beginning to lift you could probably make it worse with a high power jet, but if that's the case it needs repairing anyway . . .
Unfortunately the only effective method that I have found for cleaning tubes is a scrubbing brush and a lot of effort. You can get rotary brushes that are powered by a pressure washer -- perhaps they are the answer.
Thanks for the replys. I have never heard of baby lotion treatment. Maybe thats one job i could give to the wife, and she even gets soft hands afterwards.
As far as as cleaning tubes i have used with great sucess the green pan cleaning pads, somtimes with a sponge on the other side. On the hull cream cleaner also works a treat. Both though are hard work. I will give power washing a go but it was hard going even on the hull.