I've never used a dry shampoo, but I have dealt with dogs in casts. What I did was mix up some shampoo in a spray bottle and mist them down until damp, brush them out and then mist with clear water and towel dry. It takes a while but when your desperate its worth it!
I have even bagged up a cast with an old plastic shopping bag and taped it shuts with packing tape and bathed. No water go into or onto the cast and the dog was fine.
I have even bagged up a cast with an old plastic shopping bag and taped it shuts with packing tape and bathed. No water go into or onto the cast and the dog was fine.