Most of the 'Merlin' engined home build cars that I'm aware of that have existed have not actually been Merlin engined. They have been Meteor powered rather than Merlin powered. The Meteor being better suited to this purpose, being the non-supercharged version of the Merlin made from less exotic and cheaper materials for use on Tanks and other armoured vehicles.
However, in the USA, supercharged RR and Packard Merlins have been trashed a plenty, in both Tractor Pulling events and Hyroplane racing over the years.