I am thinking of using a strop round the mast to attach the puller, using 4mm Excel D12. I will need to put a small clip on the back of the mast to stop the strop moving down under tension. The question is will I weaken the mast by drilling and putting two rivets in the back of the mast just above the gooseneck?  I believe this area is under a significant amount of stress (or am I wrong?).

Whats wrong with the normal method of attachment? I've not seen one break there that can be attributed to fittings.

However if you're only using the eyestrap to locate a fitting on the back of the mast then glue it on with either epoxy or preferably an acrylic adhesive such as plexus. make sure you rough up the fitting well first!

Cant you just use the gooseneck itself? The strop around the mast will locate neatly using the gooseneck.