Leave it to Eric Schaeffer to vault established boundaries for Signature Theatre’s fifth world premiere of the season, “Brother Russia.” The rock musical about the mystical charlatan who refused to die features music by Dana Rowe with book and lyrics by John Dempsey, the team behind two earlier Signature hit musicals, “The Fix” and “The Witches of Eastwick.” The cast is headed by local favorites John Lescault, Natascia Diaz and Doug Kreeger.