Almost great minds think alike Col.
I went to where I used to work and scrounged the shortest tubeless valve they have (1"), took the valve out drilled a 11.5mm hole in the drum above the main cap. I got a piece of wire and bent the end to stop the valve sliding off the end. I pushed the end of the wire out through the 11.5mm hole ready.
Applied CA glue to the rubber and a spray of accelerator to the drum. Then pulled the valve up through the hole and screwed on a metal valve extension to which I attached a pair of grips and pulled the valve rubber through the side of the drum. Removed the extension and the piece of wire and job done.
Just needs somewhere to store the cap when removed for use ............
x
Tony.