wellygary:

 

"Don’t worry about trying to screw into studs behind the boards - this approach will be plenty strong enough if done competently."

 

If you can guarantee that it will only be used as a handrail- yes 

 

But at some point in the lifetime that it is installed  a 10 year old will decide that its actually designed for free hanging on,

 

I'm not sure you really want 30 kg of dynamic load on two screws in 20mm weatherboards?

 

Agreed. There's really no downside about screwing through the weatherboards into the studs so why wouldn't you? If I tripped and grabbed a handrail I would rip it out of weatherboards, or at least flex it considerably. If it's screwed through into studs with decent stainless screws it won't go anywhere.

There's no way I'd be relying on the weatherboards for strength.