I was actually in Wellington Barracks a few weeks ago when it was sewn onto Ian Mott's tunic. Approval was given for the Garrison Sergeant Major to wear the RSM rank badge on top of four chevrons, as was done in the Victorian era. The badge is a two part badge using a drum major's four chevron badge inverted, with the normal Guards RSM badge sewn onto it. It is very impressive.
There are no plans to make a single piece badge for this, it will continue to be sewn together from separate chevrons and royal arms.