Hello there Barrie
Here's an idea, which I have used before in the past - for planking where you do not wish to have any pins/holes in the planking.
You construct 3-4 of these square 'picture frames' , so that they slip over your hull; you space them out roughly about 2 inches apart. You then secure them onto some wooden battens on the bottom of the frames.
The frames are then held vertical and square and will not move. You slide your hull through the centre; as per drawing - and on the opposite side to which you are going to plank, you put packing pieces in.
Then, you may proceed to put your plank onto the hull and hold it in place with a pusher stick, which is clamped to the 'picture frame'. Obviously, the more 'picture frames' you use the better, and this will also allow you to put a plank on wet.
Word of caution though; if you have removed your deck support frames from the ribs; you may have to put 'temporary' strengthening across the width of the hull on the inside.
Hope this is of some help.
aye
john e
bluebird