If it is an improved chamber, then by effectively firing 'fire-forming' loads you could run the risk of eroding the chamber throat. I think I would go the route of getting a Hornady Hydraulic Forming Die (to mimic the fire-forming) and then some custom full length loading dies. You shouldn't have any difficulty getting hold of custom dies from the USA if you get a chamber cast done and send off the casts (or send the fired cases - a RFD should be able to help there).
At the end of the day, its a Westley Richards DR - presumably fairly valuable, so I personally wouldn't be taking any short cuts. What do others think?